add UFCS/MCS cast, and addrCast

[timotheecour]

MCS cast

I use it a lot in my code, it leads to code that's easier to read/write/edit than cast[F](expr) especially in more complex expressions; yet it's as easy to audit than builtin cast (eg when grepping for cast).

for reference, previous attempt was in std/sugar (nim-lang/Nim#8528), I'm now moving this to fusion/pointers which is a module dedicated to making unsafe code easier to handle.

example

see runnableExamples

addrCast

it's a common pattern, and std/pointers is a good fit for it
see runnableExamples