Check whether loans conflict with old loans or with themselves.

Along the way, convert from dvec-of-dvec representation to track loans in scope
to just a single flattened list.  It's more convenient.

Fixes #3765. r+ pcwalton.

src/test/compile-fail/borrowck-loan-local-as-both-mut-and-imm.rs

@@ -0,0 +1,25 @@
+use core::either::{Either, Left, Right};
+
+    fn f(x: &mut Either<int,float>, y: &Either<int,float>) -> int {
+        match *y {
+            Left(ref z) => {
+                *x = Right(1.0);
+                *z
+            }
+            _ => fail
+        }
+    }
+
+    fn g() {
+        let mut x: Either<int,float> = Left(3);
+        io::println(f(&mut x, &x).to_str()); //~ ERROR conflicts with prior loan
+    }
+
+    fn h() {
+        let mut x: Either<int,float> = Left(3);
+        let y: &Either<int, float> = &x;
+        let z: &mut Either<int, float> = &mut x; //~ ERROR conflicts with prior loan
+        *z = *y;
+    } 
+
+    fn main() {}