reduced call overhead (and code size) in the parser by changing suitable cpdef functions into plain cdef

optimise 'int_val in string_literal' into a switch statement when x is an arbitrary expression, not only a name

Until the dust settles, this is better than manually patching Sage's Cython.

make IntNode smarter about the type of integer it chooses for its value, promote plain >=32bit literals to Python objects to make sure their values pass through the C code