[SAoC 2022] Replace druntime Hooks with Templates: Milestone 3, Weeks 1 & 2

[rikki cattermole]

Hopefully this work will mean that some code will be removed from rt.

That bunch of code is a real problem.

Right now no one can implement a GC without compiling it into druntime 
or reimplementing stuff in rt. Not a great situation!

It also contains my most disliked set of hooks, memset as they could be 
very easily removed entirely.