rather than a constant that is 2**X. Reorder them to put bits for
operands that have symbolic names in the upper bits, so they can
be packed into an int where the lower bits contain the value that
corresponds to that symbolic name.
(predefined_symbo): Add struct.
(tic80_predefined_symbols): Declare array of translations.
(tic80_num_predefined_symbols): Declare size of that array.
(tic80_value_to_symbol): Declare function.
(tic80_symbol_to_value): Declare function.
(FMT_UNUSED, FMT_SI, FMT_LI, FMT_REG): Delete.
(TIC80_VECTOR): Define a flag bit for the flags. This one means
that the opcode can have two vector instructions in a single
32 bit word and we have to encode/decode both.
