Add a second example for documentation purposes

Update README.md and TODO.md

cmp_read_str now adds a terminating NULL if there is room

The cmp_read_*ext* functions either disappeared or I never wrote them,
so I rewrote them

Refactored all the cmp_read_* functions to use cmp_read_object

Removed set_error, all it did was set cmp_ctx_t.error

Changed sizes from 64-bit to 32-bit; 64-bit was unnecessary

Added documentation comments to cmp.h