Fix parsing oct and hex numbers

[qzb]

Fixes #363

[geseq]

LGTM

[d5]

Thanks for the PR. When I tested the following code at the VM-level:

expectRun(t, `out = 1 + 010`, nil, 9)    // works
expectRun(t, `out = 1 + 0x10`, nil, 17)  // works
expectRun(t, `out = 1 + 0o10`, nil, 9)   // Parse Error: expected ';', found o10
expectRun(t, `out = 1 + 0b10`, nil, 3)   // Parse Error: expected ';', found b10

I think we should fix this too if we want to fully support the binary/octal numbers?

[qzb]

@d5 I've rewritten numbers scanning from scratch. Instead of trying to scan only valid numbers, it scans for things resembling number and let strconv do the validation. It should be fully compatible with golang's int and float literals.

If you don't like those changes, I can revert them and add support only for literals you mentioned.

[geseq]

Nice one!

[geseq]

@d5 will need your approval as well