feat: optimized the code (for..let loops over for..of, faster string join algo, etc.)

[ncpa0cpl]

Added code optimizations:

1. Instead of for..of loops that rely on iterators used the good ol' for..let i = 0; loops which are much faster
2. Replaced all usages of String.join() with a much faster custom implementation
3. Reduced the amount of needless object instantiations, (there were some places where a new object or array was created for convenience reasons, but was not really necessary) - this should slightly reduce the required GC time for cases where a lot of JSX is being processed.
4. JSX Elements props and children are made immutable up-front via Object.freeze