`ToBigInt` and `ToBigUint` should not return an error

[lifthrasiir]

It is a side effect of #9250, which added those traits as a special case of the FromPrimitive trait. This is rather strange, since the current ToBigInt implementors do not fail at all ( ToBigUint does fail, but only because they are implemented for signed integers as well).

Given that BigInt and BigUint already implements FromStr (that would correspond to impl<'a> ToBigInt for &'a str and so on), it would be better to make them implemented only for types that would definitely convertible to the big integers without an error. More precisely:

• Make ToBigInt::to_bigint and ToBigUint::to_biguint return BigInt and BigUint respectively.
• Remove implementations of ToBigUint for signed integers and BigInt.
• Add a new into_biguint (since it's more efficient, and does not confuse users of ToBigUint) to BigInt, which returns Option<BigUint>. We may also have into_bigint to BigUint, at expense of symmetry (it becomes asymmetric since it should not return Option<BigInt>).

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#336

[rust-highfive]

This issue has been moved to the RFCs repo: rust-num/num#10