Support --gprof build flag

See merge request hammer/hammer!20

h_drop_from

See merge request hammer/hammer!19

A working h_drop_from which rewrites its underlying sequence as a sequence containing some ignored parsers.

enable h_repeat_n() for CF backends

See merge request hammer/hammer!18

Test/fix h_bits(0, ...)

See merge request !17

fix string escaping in json output

See merge request !15

Support custom pretty-printers with USER token types

See merge request hammer/hammer!14

add combinators h_skip, h_seek, h_tell

See merge request hammer/hammer!13

revert changes to examples/base64 - bad merge

See merge request hammer/hammer!12

add const to the pointer arg to h_make_bytes

See merge request hammer/hammer!11

Compiler fixes

See merge request hammer/hammer!10

also allow NULL argument for unamb_sub to mean default

I think this was a mixup with issue #5. None of his test cases were actually
failing.

Fix "GLR backend reaches unreachable code"

Closes #19

See merge request hammer/hammer!8

In `h_sequence__ma` the same bug occurs, and it manifests as a crash
in the same way, so I've added a test for it.  In `h_permutation__ma`
it evidently exists in the same form, but I haven't figured out how to
reproduce it; in that case I added a fix to the implementation, but no
test.