Add fixed sized arrays for native module records

[erickt]

Some c libraries, like zeromq and libclang, return structures by value that contain fixed size arrays. We can't bind to these functions though because there is no way to express a fixed size array in a structure in rust. It would greatly simplify binding to these libraries if it were supported.

[graydon]

See also https://github.com/mozilla/rust/wiki/Proposal-for-fixed-length-arrays

[graydon]

This is underway now in #2112, closing this as a duplicate.