Support more robust integer lexing (hex, octal, and arbitrary radix)

[jeaye]

Clojure docs: https://www.clojure.org/guides/learn/syntax#_numeric_types

In particular, we need to support the following:

• Octal literals (prefixed with 0)
• Hex literals (prefixed with 0x) -- case insensitive
• Arbitrary radix (prefixed with N and then r, where N is the radix; e.g. 2r10101 for binary, 16rbadf00d for hex)

We can use std::stol for all of this, which also takes in the radix. We should check the position output param to ensure that the full number was consumed. If not, it's invalid.

[jianlingzhong]

i can take this one.