[RFC/WIP] Specify alignment of primitve types

[vchuravy]

This is my first time feeling my way around the parser code so any pointer would be welcomed. The goal is to allow users to specify alignments for bitstypes. My particular use-case comes after talking to @simonbyrne about Float128 support. I separated out the ABI changes for Float128 which will be part of a later PR.

I would want to make the alignment an optional value since their should be only a limited number of use-cases, any pointer how do to that in lisp?

[JeffBezanson]

Cool. This looks basically right, it's just a matter of deciding exactly what syntax we want. For example we might want

primitive type Float128 <: AbstractFloat 128
    alignment=16
end

to leave more room for other features in the future.

[vchuravy]

That sounds like an excellent idea. Any pointers how to do so without duplicating most of struct-def-expr- in julia-synatx.scm?

[vtjnash]

also fixes #20415

[vchuravy]

Very stale.

[simonbyrne]

I would still love a real Float128 (and perhaps Float80) one day...