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: FuelLabs/sway-rfcs#11

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

• Add the declaration engine skeleton code #2630
• Make the DeclarationEngine a lazy_static as a semi-temporary bandaid #2675
• Pass the Declaration Engine to IR gen #2642
• Add storage to the declaration engine #2643
• Add structs to the declaration engine #2631
• Add enums to the declaration engine #2632
• Add traits to the declaration engine #2633
• Add impl's to the declaration engine #2634
• Add functions to the declaration engine #2635
• Add constants to the DeclarationEngine #2679
• Add ABI declarations to the DeclarationEngine #2680
• Remove function declaration from function application AST node #2786
• Put functions inside of impl blocks inside the DE #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 🥳