Micro optimisations & perf updates

[wouterlucas]

Work in progress but this PR does:

• Reduce branching in hot paths (or attempt to clean up)
• Reduce look ups in variables, const them out earlier
• Reduce assert fn calls where we no longer need them
• Deduplicate a lot of code

Avoiding Text paths for now, as that's part of another change.

Big gains on node creation / node deletion, still investigating the update/swap cases.