optimize field access when field types don't depend on parameters

[JeffBezanson]

Example:

mutable struct RemoteChannel{T<:AbstractChannel} <: AbstractRemoteRef
    where::Int
    whence::Int
    id::Int
end

This change generates better code when we only know the type of something is RemoteChannel. It also computes a layout for the wrapper of such types, and shares it for all instances. We just need to make sure we never assume t->layout != NULL means leaftype, but I think we are pretty good about that. In any case this should be checked over carefully before merging this.

@nanosoldier runbenchmarks(ALL, vs = ":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here. cc @jrevels

[vtjnash]

We just need to make sure we never assume t->layout != NULL means leaftype

I think we should be good at this, since we already will fill in layout for other non-leaftypes such as bitstypes and singletons. I think we just need to be careful that we are aware that now T->layout will apply to all subtypes of T. Perhaps worth putting a comment to that effect somewhere? (Since it means we are now depending on the type-system always returning correct answers for isleaftype)

This may need support from dump.c to avoid making copies of them in the sysimg, but this shouldn't be any worse then we do now.

[vtjnash]

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