Fix calls to isalnum() and isalpha() to prevent negative inputs

[dechamps]

See https://en.cppreference.com/w/cpp/string/byte/isalnum:

Like all other functions from <cctype>, the behavior of std::isalnum is undefined if the argument's value is neither representable as unsigned char nor equal to EOF. To use these functions safely with plain chars (or signed chars), the argument should first be converted to unsigned char

This issue was found by the Visual C++ Static Analyzer (/analyze):

toml\toml.h(805) : warning C6330: 'char' passed as _Param_(1) when 'unsigned char' is required in call to 'isalnum'.
toml\toml.h(821) : warning C6330: 'char' passed as _Param_(1) when 'unsigned char' is required in call to 'isalpha'.
toml\toml.h(824) : warning C6330: 'char' passed as _Param_(1) when 'unsigned char' is required in call to 'isalpha'.
toml\toml.h(1304) : warning C6330: 'char' passed as _Param_(1) when 'unsigned char' is required in call to 'isalnum'.

[mayah]

lgtm