Implement FromStr for Imm64, Ieee32, Ieee64.
Implement FromStr for Imm64, Ieee32, Ieee64.

These are bitwise exact conversions from string to immediates, implementing the
inverse of the Display trait.

Only accept hexadecimal floating point numbers to avoid issues with rounding
when converting decimal numbers to binary.

stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,stoklund/cretonne,sunfishcode/cretonne