Handle very large hexadecimal number literals correctly

Very large decimal number literals, binary number literals and octal
literals are lexed into an INFINITY token (instead of a NUMBER token)
and compiled into `2e308`. That is is supposed to be the case for very
large hexdecimal dumber literals as well, but previously wasn't.

Before:

    $ node -p 'require("./").tokens(`0x${Array(256 + 1).join("f")}`)[0][0]'
    NUMBER

After:

    $ node -p 'require("./").tokens(`0x${Array(256 + 1).join("f")}`)[0][0]'
    INFINITY

This commit also cleans up `numberToken` in lexer.coffee a bit.