normalize for Fail printing locations

See merge request !334

std++ 1.6.0 release notes

Closes #119

See merge request !333

countable.v: prove choose_proper

See merge request !327

Noticed while working towards "Pragmatic quotients".

No changes to the contents.

base.v: fix typo in comment about MonadSet, and extend

See merge request !332

Mark set_size and set_fold as TC opaque

See merge request iris/stdpp!329

Normalize focused goal output

See merge request !330

Merge the "1 focused goal" line with the subsequent "(shelved: 1)" line,
since this is the new output in Coq 8.15+.

std++ does not currently produce this output, since no test calls `Show`
with shelved goals, but this future-proofs the test normalization.

fin_sets: add missing Params instance for `set_map`

See merge request !328

Noticed from VERY slow rewrites.

f_equiv optimizations

See merge request iris/stdpp!325

f_equiv: slightly better support for function relations

See merge request iris/stdpp!326