Overview

This change adds PinnedByteArray support to Unison, enabling efficient pinned memory management for low-level I/O operations and system calls. Previously, Unison only had MutableByteArray for mutable byte operations, but lacked the ability to create pinned arrays that prevent the garbage collector from moving memory during critical operations.

Old behavior: Users could only work with regular mutable byte arrays that could be moved by the garbage collector, making them unsuitable for system calls, certain I/O operations, or FFI that require stable memory addresses.

New behavior: Users can now create pinned byte arrays using e.g. IO.pinnedByteArray ensuring memory remains at a fixed address during critical operations. They can be cast to mutable arrays using PinnedByteArray.cast, so wherever a MutableByteArray can be used, a PinnedByteArray can also be used after casting. Operations that require pinned memory use PinnedByteArray directly. Crucially, a MutableByteArray cannot be cast to a PinnedByteArray, which otherwise would cause segfaults.

Example usage:

-- Create a pinned byte array
pinned = IO.pinnedByteArray 1024

-- Cast to mutable array for read/write operations
mutable = PinnedByteArray.cast pinned

-- Write data (memory stays pinned)
write8 mutable 0 42
write16be mutable 1 12345

-- Read data back
value1 = read8 mutable 0  -- Returns 42
value2 = read16be mutable 1  -- Returns 12345

This is particularly useful for:

• Network operations that require stable buffer addresses
• System calls that need pinned memory
• Future FFI operations using external C libraries
• Performance-critical I/O where memory movement is unacceptable

Fixed buffer I/O

This PR also adds Socket and Handle I/O operations that use pinned memory:

• IO.socketSendBuf - Sends a number of bytes from a PinnedByteArray to a Socket.
• IO.socketReceiveBuf - Blocking read of some number of bytes from a Socket into a PinnedByteArray.
• IO.putBuf - Writes a number of bytes from a PinnedByteArray to a Handle.
• IO.getBufSome - Reads whatever bytes are available on a Handle to a PinnedByteArray. Waits only if no bytes are available.
• IO.fillBuf - Waits until enough bytes are available on a Handle to fill a PinnedByteArray, then reads them into the array.

Test coverage

Operations on pinned arrays are exercised via transcripts.