Tex pool reorg

[brucemiller]

Split the (huge) TeX.pool into more manageable, modular components:

• TeX primitives, grouped according to David Bausum's "families" of primitives
• LaTeXML-specific utilities, helpers, etc
• LaTeXML binding for plain.tex.

Future (or continuing) work will

• regularize all LaTeXML-specific control-sequences to start with \lx@
• support reading plain.tex and latex.ltx directly for the bulk of definitions, after establishing what needs LaTeXML-specific binding, eg for semantics.

[brucemiller]

@dginev here's another take, per our discussions; give it another look.

[dginev]

✅ I like the new naming scheme. Looks good to merge.