Simplify parsing numeric arguments with .scan

[lichray]

Limitations due to portability :

1. This patch uses from_chars for parsing integer numbers. To do so, I bumped required GCC version to 8.x and required Clang/libc++ version to 7.x.
2. This patch emulates from_chars behavior for parsing floating-point numbers with strtod, but this function is locale-aware.
3. The emulation does not use strict grammar to distinguish fixed and scientific format. As a result, NAN(char_sequence) (signal NaN, but should be parsed as quiet NaN) may fail to parsed.

Caveats:

1. There is no base == 0 case for from_chars; integer format detection for 'i' shape is done by hand.
2. Hexfloat is not considered possibly negative as positional argument, so negative hexfloat currently is not a valid input.
3. + sign is not allowed, but this is a result of sticking with from_chars's grammar.
4. We throw range_error for ERANGE and std::errc::result_out_of_range, unlike stoi and stod which throw out_of_range. Our choice is correct here.

fixes: #63