Increased TInstr field sizes: allow long jumps and 65535 VM registers

[zevv]

Increases range from from 32K to 8M instructions. Leaves plenty of space in TInstr that can later be used for other purposes.

Fixes #12727, supersedes #12742

[Araq]

Can we get some benchmark numbers? How much memory is wasted? I expect the number of VM instructions in the thousands so we lose about 10_000 * 4 = 40K bytes. Not really significant. And why not also increase the number of registers we have?

[zevv]

I split my inital commit into smaller functional pieces because discussions were diverging to places that were not yet relevant imho. This PR only scratches my itch, the next PR after this will increase A,B and C sizes to 16 bits to allow more registers. That would leave 8 bits unused only, so not really space for another register - but I'd like to save discussions about this for the next PR, this one is only about increasing Bx.

[zevv]

• Memory: when running a hefty npeg compiler I end up with tens of thousands of instructions, so that's in the tens-of-kilobytes range.
• Performance: My tests consists of timing the nim vm/ test suites, for which I don't see any significant impact over the noise.

I'd be happy to do more thorough testing, but I'd needs some hints on what to test.