Both /.gitignore and /lib/coq/.gitignore ignored some files in /lib/coq.
This commit removes /lib/coq/.gitignore and moves all ignore-statements
to /.gitignore . This should simplify the maintenance of gitignore files.

The situation with /test/mono/.gitignore is analogous.

The "c" tests produce some *.h files that didn’t get ignored. The "coq"
tests also produce some files that weren’t ignored.

SMT seems sensitive to gensym counter being reset between definitions,
but it shouldn't care due to unique_per_function_ids... need to
investigate further. Only causes a single test to fail so must be
subtle. Diffing between the bad/good versions reveals a few lines of
generated SMT go missing when the gensym counter is reset.

Make it so that jib_compile.ml never relies on specific string encodings
for various constructs in C. Previously this happened when
monomorphisation occured for union constructors and fields, i.e.

x.foo -> x.zfoo_bitsz632z7

Now identifiers that can be modified are represented as (id, ctyp list)
tuples, so we can keep the types

x.foo -> x.foo::<bits(32)>

This then enables us to do jib IR -> jib IR rewrites that modify types

In particular there is now a rewrite that removes tuples as an IR->IR
pass rather than doing it ad-hoc in the C code generation, although this
is not on by default

Note that this change seems to have triggered an Ott bug so jib.lem is
now checked in and not generated from Ott