Build strings without repeated re-allocation and copies

Same problem as #35, but difference is that once strings/Atoms are generated, they're immutable. So there's a simpler solution:

Assume we're going to create our own arena allocator.

Fill the allocator from both ends:

• Fill from end for AST nodes (bumping downwards, same as bumpalo does).
• Fill from start for strings (bumping upwards).

Same sort of idea as how heap grows upwards, and stack grows downwards.

We only build 1 string at a time, so can always grow strings in place while building them. This avoids memory fragmentation, and unused gaps in arena.

Side benefit: All string data in one place means AST transfer can convert all strings from UTF8 to UTF16 in one go (still complicated due to needing UTF16 indexes to slice that string, but it's a step in right direction).

Read issue #35 first, then assess the proposed two-ended arena allocator: AST nodes grow downward and strings upward while one string is built at a time. Done means avoiding repeated string re-allocation and copies while supporting the stated UTF-8 to UTF-16 transfer goal.