use a Redblackmap keyed on the constant's name

rather than a term net keyed on the constant.
move labels-specific stuff into a separate lib

to avoid having to recompute the main bootstrap theorem whenever this
code changes
add EVAL theorem for code_labels on the code for call_repl_step
bug fix: only want the Lab clauses of inst_labels's definition

now that we match on the constant's name rather than the actual term,
it's possible to get the clause for Jump(Addr n) rather than Jump(Lab n)
(matching on "Jump") - but we never want the former; so filter it out.
bug fix: deconstruct the term properly in inst_labels_conv

in order unfold the map's abbreviation and apply ffconv to the FLOOKUP.
this is protected by a TRY_CONV so went through OK before, but it means
the FLOOKUPs were never simplified. this version, which might be correct
now, might therefore take a much longer time to run...