Added integer base conversions to str.to_int() and int.to_string()

Meson strings can now be converted from hexadecimal, octal, and binary integer literals with str.to_int(). Integers can also be formatted back to strings in those bases with int.to_string(format:).

assert('0xff'.to_int() == 255)
assert(255.to_string(format: 'hex') == '0xff')