An external type represents the type of external objects.

Instance variables:
compiledSpec <WordArray> Compiled specification of the external type
referentClass <Behavior | nil> Class type of argument required
referencedType <ExternalType> Associated (non)pointer type with the receiver

Compiled Spec:
The compiled spec defines the type in terms which are understood by the VM. Each word is defined as:
bits 0...15 - byte size of the entity
bit 16 - structure flag (FFIFlagStructure)
This flag is set if the following words define a structure
bit 17 - pointer flag (FFIFlagPointer)
This flag is set if the entity represents a pointer to another object
bit 18 - atomic flag (FFIFlagAtomic)
This flag is set if the entity represents an atomic type.
If the flag is set the atomic type bits are valid.
bits 19...23 - unused
bits 24...27 - atomic type (FFITypeVoid ... FFITypeDoubleFloat)
bits 28...31 - unused

Note that all combinations of the flags FFIFlagPointer, FFIFlagAtomic, and FFIFlagStructure are invalid, EXCEPT from the following:

FFIFlagPointer + FFIFlagAtomic:
This defines a pointer to an atomic type (e.g., 'char*', 'int*').
The actual atomic type is represented in the atomic type bits.

FFIFlagPointer + FFIFlagStructure:
This defines a structure which is a typedef of a pointer type as in
typedef void* VoidPointer;
typedef Pixmap* PixmapPtr;
It requires a byte size of four (e.g. a 32bit pointer) to work correctly.

[Note: Other combinations may be allowed in the future]

	converting
	asNonPointerType asPointerType

	printing
	printOn:

	private
	compiledSpec: embeddedSpecWithSize: externalTypeName headerWord newReferentClass: readFieldAt: setReferencedType: writeFieldAt:with:

	testing
	isAtomic isIntegerType isPointerType isSigned isStructureType isUnsigned isVoid

	housekeeping
	cleanupUnusedTypes noticeModificationOf: noticeRemovalOf: noticeRenamingOf:from:to:

	instance creation
	new

	private
	atomicTypeNamed: forceTypeNamed: newTypeNamed:force: pointerSpec structTypeNamed: structureSpec

	type constants
	bool byte char double float long sbyte schar short signedByte signedChar signedLong signedLongLong signedShort string ulong unsignedByte unsignedChar unsignedLong unsignedLongLong unsignedShort ushort void

	accessing	top
	atomicType
	byteSize Return the size in bytes of this type
	compiledSpec Return the compiled spec of the receiver
	referentClass Return the class specifying the receiver

	converting	top
	asNonPointerType convert the receiver into a non pointer type
	asPointerType convert the receiver into a pointer type

	printing	top
	printOn: Append to the argument, aStream, a sequence of characters that identifies the receiver.

	private	top
	compiledSpec:
	embeddedSpecWithSize: Return a compiled spec for embedding in a new compiled spec.
	externalTypeName
	headerWord Return the compiled header word
	newReferentClass: The class I'm referencing has changed. Update my spec.
	readFieldAt: Return a string defining the accessor to an entity of the receiver type starting at the given byte offset. Private. Used for field definition only.
	setReferencedType:
	writeFieldAt:with: Return a string defining the accessor to an entity of the receiver type starting at the given byte offset. Private. Used for field definition only.

	testing	top
	isAtomic Return true if the receiver describes a built-in type
	isIntegerType Return true if the receiver is a built-in integer type
	isPointerType Return true if the receiver represents a pointer type
	isSigned Return true if the receiver is a signed type. Note: Only useful for integer types.
	isStructureType Return true if the receiver represents a structure type
	isUnsigned Return true if the receiver is an unsigned type. Note: Only useful for integer types.
	isVoid Return true if the receiver describes a plain 'void' type

	class initialization	top
	initialize ExternalType initialize
	initializeAtomicTypes ExternalType initialize
	initializeDefaultTypes ExternalType initialize
	initializeFFIConstants ExternalType initialize
	initializeStructureTypes ExternalType initialize
	obsolete Change the receiver and all of its subclasses to an obsolete class.

	housekeeping	top
	cleanupUnusedTypes ExternalType cleanupUnusedTypes
	noticeModificationOf: A subclass of ExternalStructure has been redefined. Clean out any obsolete references to its type.
	noticeRemovalOf: A subclass of ExternalStructure is being removed. Clean out any obsolete references to its type.
	noticeRenamingOf:from:to: An ExternalStructure has been renamed from oldName to newName. Keep our type names in sync.

	instance creation	top
	new Use either the type constants or #externalType for creating external types

	private	top
	atomicTypeNamed:
	forceTypeNamed:
	newTypeNamed:force:
	pointerSpec
	structTypeNamed:
	structureSpec

	type constants	top
	bool
	byte byte defaults to unsigned byte
	char char defaults to unsigned char
	double
	float
	long
	sbyte
	schar
	short
	signedByte
	signedChar
	signedLong
	signedLongLong
	signedShort
	string
	ulong
	unsignedByte
	unsignedChar
	unsignedLong
	unsignedLongLong
	unsignedShort
	ushort
	void