Custom strtod implementation

A custom implementation of strtod which functions very similar to the standard C strtod function with some minor differences:

1. this function skips over underscores to allow for underscore seperated numbers
2. by default, errno is not set to ERANGE when the normalized value of the number would be less tha DBL_MIN but is instead set if the resulting double would be 0 where the value is not 0. (see below)
3. where strtod can check for the 0x prefix tp parse hexadecimals, c8_strToD does not by default. this behavior can optionally be enabled with a compiler flag. (see below)
4. when enabled c8_strToD checks for any prefix (0x, 0o, 0b) and calls c8_strToLL.
   1. Note that this is not the same behavior strtod does. strtod only seems to handle prefixes 0x and 0X. Also a number 0x will be consumed as 0 with x remaining while c8_strToLL would not consume any bit of that number at all.

There may be more inconsistancies. These are the minor ones due to design, simplicity, or what I have found through testing...

Options

There are some togglable options to change the parsers behavior. they can be enabled with define's. Here are some options and what they do:

Option	Description	Examples
USE_STRTOLL_FOR_PREFIX	Allows the parser to to send numbers beginning with 0x, 0o, and 0b to c8_strToLL to parse as a number of a different base. (note that this is currently included using a relative path)	numbers like 0xABCDEF, 0x10101, and 0o1234567 will be parsed
USE_DEFAULT_STRTOLL	(depends on USE_STRTOLL_FOR_PREFIX to be enabled) sends numbers to the default C strtoll function instead of the custom one
ERRNO_AT_DBL_MIN_AND_MAX	Sets errno when a number whose normalized value is greater than DBL_MAX or less than DBL_MIN.