The istring and string_view operators have identical implementations. By
changing the istring operators to cast to string_view and use that
implementation instead, the number of redundant implementations is
reduced. This does incurr a small performance penalty, around 15 cycles
when tested on the MGBA Gameboy Advance emulator (uses an armv7tdmi).
When compared to the time operations take, the performance difference is
negligible. Ex. An insertion with two 8 character strings takes around
450 cycles.
