This PR ties up loose ends in the implementation of several I/O buffer operations by adding proper length parameters and bounds checking, making them safer and more consistent with established patterns.

Namely:

• Updated IO.fillBuf.impland IO.getBufSome.impl to accept a length parameter: Handle → PinnedByteArray g → Nat → {IO} Nat. Added bounds checking. This is to make these operations more flexible and useful in practice. I realized that making the primitive operations only ever completely fill the buffer makes it hard/dangerous to write abstractions on top of these.
• Fixed withMutableByteArrayContents helper function to properly use PA.keepAlive with the mutable array instead of just its pointer. Using just the pointer is the same as doing nothing, since it is not a GC root.
• Added comprehensive tests covering fillBuf, putBuf, and getBufSome operations.