New VM instruction format

VM instructions are now variable size; one or two 32 bit words.

The first instruction word holds an 8 bit opcode, an 8 bit register
index and a 16 bit general index, used for register index, subvoice
ID and similar, depending on instruction.

Some instructions have a second 32 bit word, which then holds a
16:16 signed fixed point value. (Same format as the VM registers,
that is.) This eliminates the f20 floating point format previously
used for immediate fixed point values.

Some additional range checks were added in the compiler, including
all branch fixups, which now use the new inline a2c_SetA2() instead
of directly manipulating instructions.

Closes #64.