Implement "@nogil" as decorator in pure Python mode

[scoder]

Implement this:

@cython.cfunc
@cython.nogil
def func() -> cython.int:
    return 1

• Add nogil to the list of directives in Cython/Compiler/Options.py.
• In the AdjustDefByDirectives tree transformation, apply the nogil directive wherever the nogil modifier is allowed, i.e. cdef/@cfunc and cpdef/@ccall functions.
• Wherever decorators are allowed, but nogil is not, reject it explicitly (because it is no longer rejected by the parser).
• Write tests for the good cases and for invalid cases.
• Optionally: allow @nogil on Python (def) functions and interpret it as with nogil.

Also see #2579.