Allow binding of nested patterns

See src/test/run-pass/nested-patterns.rs for some examples. The syntax is

    boundvar@subpattern

Which will match the subpattern as usual, but also bind boundvar to the
whole matched value.

Closes #838

src/test/run-pass/nested-patterns.rs

@@ -0,0 +1,12 @@
+fn main() {
+    alt {a: 10, b: @20} {
+        x@{a, b: @20} { assert x.a == 10; assert a == 10; }
+        {b, _} { fail; }
+    }
+    let x@{b, _} = {a: 10, b: {mutable c: 20}};
+    x.b.c = 30;
+    assert b.c == 20;
+    let y@{d, _} = {a: 10, d: {mutable c: 20}};
+    y.d.c = 30;
+    assert d.c == 20;
+}