Improve String.map and String.mapi

[ospencer]

We currently implement these via String.explode and String.implode, but should instead iterate and build a new string. We probably don't want to make String depend on Buffer, but we could guess the next string size and realloc/copy if necessary.

[spotandjake]

It may make sense to implement an internal simplified version of the buffer library for this and #2087

[spotandjake]

We currently implement these via String.explode and String.implode, but should instead iterate and build a new string. We probably don't want to make String depend on Buffer, but we could guess the next string size and realloc/copy if necessary.

Instead of using a buffer for this would we be able to allocate a string of size 4*charCount which is the max bytesize the output string could be and, do the mapping and then shrink the strings bytelength to the real size without doing a realloc? This wouldn't hurt memory allocation in any way right but it would mean strings use more memory then they need beause we cant free a portion of a block right? I think this would be way more efficent then a buffer as it means we can do one alloc and write the characters directly to the string without needing any copying. Though it might result in a lot of wasted memory space if you are mapping large strings.