[parsing] limit the scope of a cut (~) to the nearest choice

[apalala]

As it was, a cut (~) could scape to calling rules instead of being limited to the enclosing choice, and that produced unexpected outcomes.

This set of changes limit the scope of cut to the nearest choice.

For example, this grammar would always fail independently of the input, when it is expected that rule two has a chance, and it always succeeds with "something" as output.

GRAMMAR = '''
    start =
        | one
        | two
        ;

    one = 
        | ~ !()   # cut fail
        | 'abc';
        
    two = `something` ; # result is the quoted text
'''

ast = parse(GRAMMAR, 'abc')
print(ast)