more signature work, fail on lhs values, accept rhs values and types

fix some more potion_type_char types,
the sig tuple is still reverse (see -Dv for registers)
rename MESSAGE to MSG (ast and op)
signatures: improve and refactor sig_compile

add TAP test via TEST_VERBOSE=1 bin/potion-test
add more signature tests
move enum PN_AST to potion.h to help gdb
add potion_sig_string and use it throughout
add potion_syntax_error helper
add more PN_STR helpers
add DBG_c and -Dc support