syntax: support multiprecision integer literals

[alandonovan]

Currently the scanner refuses to parse integer literals that can't be represented as int64:

$ skylark 
Welcome to Skylark (github.com/google/skylark)
>>> 1234567890123456789
1234567890123456789
>>> 12345678901234567890
... 
<stdin>:1:1: invalid int literal

The solution is to change the syntax.Literal.Value to allow string, int64, or *big.Int, and change the interpreter accordingly.

Also, the comment next to the Value field is wrong: it says int, not int64.

[alandonovan]

Fixed by #58.