Move _Py_Mangle from compile.c to symtable.c

[iritkatriel]

_Py_Mangle doesn't need to be in compile.c (which is > 10K lines and needs to be reduced).

Moving it to symtable.c will
(1) remove one reason for symtable.c to depend on compile.c
(2) make typeobject.c depend on the much smaller symtable.c instead of on compile.c
(3) make compile.c a little smaller.

Linked PRs

• gh-102371: move _Py_Mangle from compile.c to symtable.c #102372