[Consistency Review] BigInt::from_string should raise or add a @string.parse_bigint

[hackwaly]

Current behaviors

There's only BigInt::from_string, it will panic if input is malformed. It's impossible to catch if input is malformed.

Expected behaviors

We should have a @string.parse_bigint or make BigInt::from_string raises.

Justification

[hackwaly]

Motivation: redis RESP3 protocol bigint parsing.