Add a declaration engine

[mohammadfawaz]

The declaration engine allows the compiler add a layer of separation between AST nodes and declarations.

The goal of adding the declaration engine is to:

1. unlock trait constraints
2. stop inlining function bodies during type checking
3. perform optimizations during monormorphization

See the relevant RFC here: https://github.com/FuelLabs/sway-rfcs/pull/11

The prototype can be used to guide implementation: https://github.com/emilyaherbert/declaration-engine-and-collection-context-demo/tree/master/de

• [x] #2630
• [x] #2675
• [x] #2642
• [x] #2643
• [x] #2631
• [x] #2632
• [x] #2633
• [x] #2634
• [x] #2635
• [x] #2679
• [x] #2680
• [x] #2786
• [x] #2784

[emilyaherbert]

This is now done! 🥳

I've moved where we are tracking the monomorphization cache into this issue: #1819

[mohammadfawaz]

This is now complete, hopefully for real this time 🥳