Support more integer type conversions

[Yurunsoft]

No description provided.

[Yurunsoft]

@d99kris This PR provides support for the following types: int8_t, uint8_t, int16_t, uint16_t.

[d99kris]

Hello @Yurunsoft - thanks for contributing! Generally the changes look good. I would prefer to minimize change to existing test code lines though. And for ToStr() I believe there's an issue for signed char and unsigned char - default iostream will output them as characters (not numbers), so we need separate handling similar to the floating-point types (but just casting to int before print). Finally I'd like to add a test case that covers that (ToStr / Set for all datatypes).

I'll merge the PR to an integration branch and make the above mentioned changes, and add a message here when it's fully integrated to rapidcsv master.

[d99kris]

This has been integrated to rapidcsv master in commit 9b660bc.