Major overhaul of CRAM code (originally imported from Samtools/htlib
branch, and further back from Staden Package io_lib).

All uses of io_lib's HashTable and dstring_t types have been replaced
by khash and kstring.

Renamed io_lib subdir to cram subdir.

Needs exhaustive testing still, but compiles and can encode to and
from CRAM for one small test file.