Add support for Objc blocks (#325)

Adds new type, objc.Block, which is an objc.ID referencing an
Objective-C "block" function pointer.

Adds methods to create a Block from a Go function value,
get a Go function value from a block, directly invoke
a block function, and handle Objective-C memory management
(e.g. Copy/Release).

Mitigates pressure on purego Callback limit by relying on the
fact the first argument passed to a block implementation is the
block itself. This allows for a single callback to handle every
block instance that has the same signature, by way of keeping
an association between the Go func value and the block instance
it was used to create.

Was refactored from code in a different personal project to
better fit the purego convetions and architecture.

Closes #129

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Co-authored-by: James Welch <[email protected]>

objc: bug fix: wrong implementation in encodeType for a struct (#324)

Updates #225

objc: implement ability to create Protocols at runtime (#320)

Closes #321

internal/fakecgo: disable race in fakecgo (#319)

Closes #318

internal/fakecgo: disable race in fakecgo (#319)

Closes #318

Allow uintptr in struct on amd64 (#317)

all: fix the link error with Go 1.24.3 (#312)

This addresses the issue golang/go#73617.

Closes #313

all: fix the link error with Go 1.24.3 (#312)

This addresses the issue golang/go#73617.

Closes #313

Fix alignment of stack args on macOS (#310)

Fixes #309

.github/workflows: update Go versions for FreeBSD (#296)