Undetected by tests: `hGetBufExactly` and `hGetBufExactlyAt` read too many bytes in the presence of partial reads

[jorisdral]

#98 fixes a bug in hGetBufExactly and hGetBufExactlyAt related to partial reads, but this test failure was not caught by our family of propGetterGetsAll tests. We should find out why these tests didn't fail.

cabal run fs-sim-test -- -p "propGetterGetsAll"

The bug was that, in the presence of partial reads, these hGetBuf* functions would try to read more bytes than requested, instead of accounting for the number of bytes we had already read.