The fastest, constant able (thanks to constcat), no-std compatible way to concatenate &strs.

Comparison with other macros

This is as fast or faster than the fastest string concatenating crates (I checked those in hoodie/concatenation_benchmarks-rs).

Those have other problems too:

• concat_string_macro evaluates expressions twice and requires std.
• concat_strs_macro doesn't work for certain expressions.
• string_concat_macro is the best, but it doesn't have the last two of the optimizations below. As a nitpick, it also requires that you use string_concat::string_concat_impl. I know, I know. Grasping at straws, but I wanted to go over all the differences.

Optimizations

• Each expression gets a variable and thus won't be evaluated twice at runtime.
• If you pass multiple constants or literals in a row, they will be concatenated at compile time and pushed to the buffer only once.
• Passing only constants or literals makes the macro act as the concat! macro and a literal will be returned.