Regression: Invalid type used when allocating string, f_string_t should instead be...

Regression: Invalid type used when allocating string, f_string_t should instead be f_char_t.

I noticed odd differences between the 0.6.x and the 0.7.x in terms of memory usage.
I discovered this to be due to a bug where the f_string_t is being used when the type should be instead f_char_t.
This is likely a bug introduced in some past refactor.