protoparse: double/float constants that are larger than int64 range cause error

[jhump]

If a numeric literal, such as used to define the value for a double or float option, uses neither a dot nor scientific notation, then it will cause a parse error if it exceeds the range of a uint64. If a negative literal is used, the error happens if it exceeds the range of an int64.

These are two minimal reproducers:

// positive literal
message Foo { optional double bar = 1 [default = 18446744073709551616]; }

// negative literal
message Foo { optional double bar = 1 [default = -18446744073709551615]; }

[jhump]

Fixed in #316