scalar metadata propagation through Array

[jrevels]

This is similar to what Box already supports, but allowing for dynamic integer indices in addition to statically named fields.

[jrevels]

Okay, the hard work for this is done (see e3bbc3e).

The main action items left here are to implement the other array primitives e.g.:

• Array{T,N}
• Expr(:foreigncall, :(:jl_array_grow_beg), ...)
• Expr(:foreigncall, :(:jl_array_grow_end), ...)
• Expr(:foreigncall, :(:jl_array_grow_at), ...)
• Expr(:foreigncall, :(:jl_array_del_beg), ...)
• Expr(:foreigncall, :(:jl_array_del_end), ...)
• Expr(:foreigncall, :(:jl_array_del_at), ...)

The :foreigncall nodes can just be intercepted as part of Cassette's overdub pass when metadata boxes are enabled (like how Cassette transforms new(...) to _newbox(...)).

[jrevels]

superseded by #43