Fixes #370

Fixes the RON grammar for floats to be equivalent to FromStr's grammar (which we use internally):

float = ["+" | "-"], ("inf" | "NaN" | float_num);
float_num = (float_int | float_std | float_frac), [float_exp];
float_int = digit, { digit };
float_std = digit, { digit }, ".", {digit};
float_frac = ".", digit, {digit};
float_exp = ("e" | "E"), ["+" | "-"], digit, {digit};

Consequently, RON now also accepts +inf, -NaN, and +NaN.

In comparison with rustc, FromStr still accepts floats without a leading digit, e.g. .3 - these continue to work. The other way around, rustc accepts underscores in floats while FromStr and all other parsers I checked do not. I've added a special error FloatUnderscore to lint against this case.

• I've included my change in CHANGELOG.md