Fix incorrect overflow in UInt64.from_yaml

[HertzDevil]

UInt64.from_yaml raises YAML::ParseException if the value is larger than Int64::MAX. This is because the method relies on YAML::Schema::Core.parse_scalar which, being primarily intended for YAML::Any's use, supports only Int64. This PR fixes that by skipping parse_scalar altogether.

Note that OverflowError is never raised, since string-to-integer conversions raise ArgumentError instead, and there are no more integer-to-integer conversions.