Add to_int() stdlib function for string-to-integer conversion

[brianp]

There's no built-in way to convert a String to an Int. The package manager has a hand-rolled parse_int in aster-pkg/src/serialize.aster that does character-by-character digit matching across 30 lines of Aster.

Add to_int(text: String) throws SomeError -> Int as a builtin (like to_string(value: Int) -> String), backed by a Rust runtime function that calls str::parse::<i64>(). Should raise an error for unparseable input.