brainfuck: minor improvements to V implementation

[hungrybluedev]

Summary

• The StringIterator was almost perfect. Changed it slightly to make the code more idiomatic.
• Use enums instead of consts.
• byte to u8 conversion with v fmt

Goals

This implementation is slightly faster than the older one. At least on my machine (Ryzen 5 4500U). It should bring the V/gcc and V/clang times closer to the C versions for both bench.b and mandel.b.