Deserializing huge numbers into infinity

[alanhdu]

toml returns a NumberInvalid error when to deserialize inf, but happily serialize numbers like 99e999 into inf:

extern crate toml;
fn main() {
    let s = "key = 99e9999";
    let data: toml::Value = toml::from_str(s).unwrap();  // fails with "key = inf"
    println!("{}", toml::to_string(&data).unwrap());
}

outputs key = inf.

Given that inf is already explicitly disallowed, I feel like toml should also return a NumberInvalid error here instead of silently deserializing things into inf.

Found with rust-fuzz/targets#72

[alexcrichton]

Fixed by #183