Add semantics of the [forward] builtin.

[pgiarrusso-sl]

Like https://github.com/SkyLabsAI/BRiCk/pull/73/changes, just need to figure the right type.

Then drop -fno-builtin-std-forward from fmdeps/auto/rocq-skylabs-cpp-stdlib/tests/dune-gen.sh and fmdeps/auto-docs/content/demo/dune-gen.sh.