Revised mir-dataflow.

Incorporates many fixes contributed by arielb1.

----

revise borrowck::mir::dataflow code to allow varying domain for bitvectors.

This particular code implements the `BitDenotation` trait for three
analyses:

 * `MovingOutStatements`, which, like `borrowck::move_data`, maps each
   bit-index to a move instruction, and a 1 means "the effect of this
   move reaches this point" (and the assigned l-value, if a scoped
   declaration, is still in scope).

 * `MaybeInitializedLvals`, which maps each bit-index to an l-value.
   A 1 means "there exists a control flow path to this point that
   initializes the associated l-value."

 * `MaybeUninitializedLvals`, which maps each bit-index to an l-value
   A 1 means "there exists a control flow path to this point that
   de-initializes the associated l-value."

----

Revised `graphviz` dataflow-rendering support in `borrowck::mir`.

One big difference is that this code is now parameterized over the
`BitDenotation`, so that it can be used to render dataflow results
independent of how the dataflow bitvectors are interpreted; see where
reference to `MoveOut` is replaced by the type parameter `D`.

----

Factor out routine to query subattributes in `#[rustc_mir(..)]`.

(Later commits build upon this for some unit testing and instrumentation.)

----

thread through a tcx so that I can query types of lvalues as part of analysis.

----

Revised `BitDenotation::Ctxt`, allowing variation beyond `MoveData`.

The main motivation is to ease threading through a `TyCtxt`.

(In hindsight it might have been better to instead attach the `TyCtxt`
to each of the different dataflow implementations, but that would
require e.g. switching away from having a `Default` impl, so I am
leaving that experiment for another time.)

Author: pnkfelix

src/librustc/mir/repr.rs

@@ -226,7 +226,8 @@ pub struct UpvarDecl {
 /// list of the `Mir`.
 ///
 /// (We use a `u32` internally just to save memory.)
-#[derive(Copy, Clone, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
+#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord,
+         RustcEncodable, RustcDecodable)]
 pub struct BasicBlock(u32);
 
 impl BasicBlock {

src/librustc/mir/transform.rs

@@ -18,7 +18,7 @@ use ty::TyCtxt;
 use syntax::ast::NodeId;
 
 /// Where a specific Mir comes from.
-#[derive(Copy, Clone)]
+#[derive(Debug, Copy, Clone)]
 pub enum MirSource {
     /// Functions and methods.
     Fn(NodeId),