Rolling up PRs in the queue

Closes #15276 (Guide: if)
Closes #15280 (std::os - Add join_paths, make setenv non-utf8 capable)
Closes #15314 (Guide: functions)
Closes #15319 (Remove #[crate_id])
Closes #15327 (Simplify PatIdent to contain an Ident rather than a Path)
Closes #15340 (Guide: add mutable binding section)
Closes #15342 (Fix ICE with nested macro_rules!-style macros)
Closes #15350 (Remove duplicated slash in install script path)
Closes #15351 (correct a few spelling mistakes in the tutorial)
Closes #15352 (librustc: Have the kind checker check sub-bounds in trait casts.)
Closes #15359 (Fix spelling errors.)
Closes #15361 (Rename set_broadast() to set_broadcast().)
Closes #15366 (Simplify creating a parser from a token tree)
Closes #15367 (Add examples for StrVector methods)
Closes #15372 (Vec::grow should use reserve_additional, Vec::reserve should check against capacity)
Closes #15373 (Fix minor issues in the documentation of libtime.)