perf(semantic): avoid counting nodes

[overlookmotel]

Instead of visiting AST to count AST nodes in SemanticBuilder, just make high estimates of counts for nodes, scopes, symbols and references, based on source text length. These estimates are the maximum number of items theoretically possible for a given source text length. Almost always they'll be vast over-estimates, but should never be an under-estimate.

This ensures AstNodes, ScopeTree and SymbolTable's allocations will not grow during their construction, which is a major performance hit, but avoids the complete AST traversal we were using previously to get accurate counts.

This "estimate high" approach is a viable strategy on 64-bit systems with virtual memory, where:

1. The memory space is so large that it's almost impossible to exhaust.
2. Allocating memory only consumes virtual memory, not physical memory.

So use this faster "estimate high" method as standard, and the slower visitor-based counter on 32-bit systems and WASM.

Note: ~~I tried using shrink_to_fit on the Vecs once semantic run is complete, to free up the excess memory. But this caused the Vecs to all reallocate, which negated the performance gains.~~ It was an artefact of the custom global allocator we use in our benchmarking setup. Shrinking allocations does not cause reallocation.