[PERF] Lightweight Node Construction

[iamrecursion]

Description

As node construction is one of the most common operations in the virtual machine, it is important that it be as cheap as possible.

Currently this is quite a heavyweight task due to the boxed structure of the value tree. This could be improved.

Spec

• Come up with an artificially flat representation for these nodes. This will make allocation effectively arena-like using a vector and indices as pointers.
• Make this type safe.
• Re-engineer the value representation around this.