Use reader/writer locks for access to file handles

File handle access is currently controlled using an `MVar`, guaranteeing exclusive access to a file handle. However, this is unnecessarily restrictive: functions like `hGetSomeAt` can read from file handles without modifying the contents of the file or the current file offset.

Locate the MVar that controls file-handle access and inspect hGetSomeAt to understand which operations are read-only. Done means read-only file-handle access can proceed through shared locks while operations that modify state remain exclusively locked; the issue does not name specific files or tests.

Written by the indexing model from the issue text.