r? @brson

This fixes a problem on compiling test suits for android target.

Patch for libuv android
r? @brson

Replace the custom eq with #[deriving_eq].

All comments from previous pull requests addressed.

Follow up to #5242. All credit to @yichoi.

Conflicts:
	configure
	mk/rt.mk

This series of changes moves the representation details of algebraic datatypes (enums, and special cases like structs and tuples and (until they're fully removed) records) from various places around rustc::middle::trans into a single module, to enable future improvements in this area.

r?(@nikomatsakis), and the core developers in general; this seems like a “super-review” kind of change.

This way "field" refers to the abstraction and "element" (as in get_elt,
"get element pointer", etc.) refers to the low-level LLVM operations.