[SAoC 2021] Replace druntime Hooks with Templates: Milestone 1, Week 1

[kinke]

On Thursday, 23 September 2021 at 15:15:32 UTC, Teodor Dutu wrote:
> We skipped lowering in expressionsem.d for snippets like `S[3] 
> arr = [S(1), S(2), S(3)];`, as e2ir.d already handles such 
> cases more elegantly.

This is bad news for all non-DMD compilers, as e2ir.d is DMD 
only, and LDC and GDC would have to duplicate thew lowerings in 
the glue layer now. Please don't.