Improve handling of Unicode vs Byte strings

[Scott-Guest]

Part of runtimeverification/k#3344

Fixes the conflation between String and Bytes with regards to how escapes are handled. Specifically, now

• \xHH is always treated as U+00HH at parse time / when stored in a KOREStringPattern.
  • For Bytes, we later produce the actual char[] of bytes when the runtime token is constructed
  • This ensures KOREStringPattern::contents is always a UTF-8 encoded string and aligns with how Pyk represents Bytes (resolving Python binding for StringPattern.contents is broken #824)
• Created a separate SortCategory::Bytes and BYTES_LAYOUT
• The runtime representation for strings now uses the bit just before the length to mark whether it represents a UTF-8 encoded string or a byte string.
• We correctly print Unicode escapes (\x, \u, \U) versus individual byte escapes (only \x) depending on the sort.

A few other minor fixes along the way:

• Fixed some undefined behavior in sfprintf by ensuring va_lists are copied rather than re-used
• Renamed set_len to init_with_len because it clears also all non-length bits (got bit by this 😄)
• Re-ordered SortCategory to ensure MInt is last so that valueType.cat + valueType.bits uniquely identifies a ValueType

Note that this just fixes the infrastructural issues - a few String hook algorithms still need to be updated to give correct results with non-ASCII characters.

[dwightguth]

The code looks good but it's incomplete. I can tell you tried to update every place where we were switching on the sort category, but you didn't get them all. If you see my comment above, we are defining cmake variables corresponding to each sort category and then turning them into a macro in config/macros.h. You need to make sure you update every place where those macros occur also. I suspect if this PR were merged in its current form, it would trigger regressions in garbage collection. I would recommend trying to create a test that exercises that regression and adding it to the test suite. It should just be a matter of having a term of sort Bytes live in the configuration during a minor collection cycle.