VM: Fix chain assignment evaluation order

[schungx]

Closes #1115.

Main Issue

There is a difference between how the AST interpreter and the Grain VM handle assignment to chained expressions.

For instance, when running this script:

S[K]=b;

The AST interpreter will come back with variable 'b' not found because it evaluates the value expression first.

Rhai Grain will come back with variable 'S' not found because it evaluates the lvalue expression first.

This PR patches the VM such that it follows the AST interpreter's order and evaluates the value expression first, stashing the result into an temporary slot, processes the lvalue chain and then finally assigning the result into the chain target.

Bonus fix

Rhai sometimes checks for invariants using assert! where it knows to be true. However, the VM may be running bytecodes that are corrupted, violating such invariants.

A good example is indexers and property getters/setters. They are function calls compiled to AST with fixed numbers of arguments. Rhai's compiler will never emit an AST with the wrong numbers of arguments.

However, the VM may load an artifact that contains the wrong numbers of arguments for these function calls. There is no way for the VM to catch these cases.

Therefore the asserts have been replaced by proper errors which also resolves another fuzzing error.