Log Message:
Updated lang/coq to version 8.10.2.

Changes include:
- native 63-bit machine integers;
- a new sort of definitionally proof-irrelevant propositons: SProp;
- private universes for opaque polymorphic constants;
- string notations and numeral notations;
- a new simplex-based proof engine for the tactics lia, nia, lra and nra;
- new introduction patterns for SSReflect;
- a tactic to rewrite under binders: under;
- easy input of non-ASCII symbols in CoqIDE, which now uses GTK3.

and many small improvements and bugfixes.