Generate unicode_limits.h.

Generate unicode_limits.h.

To ensure we do not overflow a size_t while case mapping on a 32-bit
platform, we need to know the maximum amount a UTF8 string can expand.

Calculate that maximum while generating unicode tables as a part of
the update-unicode target, and output into a new header
unicode_limits.h. Minor refactoring along the way.

Add a StaticAssertDecl to check that a MaxAllocSize text value
expanded by that amount would still have a length that fits in
size_t. (We couldn't actually create a new text value out of that, but
we still need to avoid overflow.)

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3213927.1783950167@sss.pgh.pa.us