Rename htools/ to src/

Per offline discussions, this is the first patch of the
renames. Tested with "make distcheck", seems to work fine.

The only change outside of the renaming is a bit of simplification in
the .gitignore rules; otherwise, simply s/htools/src/.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>