Add tests for strings

Fix copy/paste bugs in cmp_write_bin8, cmp_write_str8, and
cmp_write_str_marker
Add tests for map and array
Add extended type writing convenience method

Update README.md
Add tests for fixed-size extended types

Fix copy/paste errors in fixed-size extended type writers

Ext types should use const void instead of just void; will probably
switch the str and bin types to this also; had to go from size => sz and
type => tp for length reasons, but that's ok

Add cmp_write_ext convenience method
Compress cmp.c a little; bugfix in cmp_write_str