Move a bunch of trans into trans_common, including the context structures.

Probably more should be moved or split off into other files. My algorithm
was something along the lines of: move the contexts and their transitive
dependencies along with some functions to work with them. I stopped when
I was going to have to start pulling glue generation, which really
should go into a trans_glue file.

Author: msullivan

src/comp/back/link.rs

@@ -2,9 +2,9 @@
 import driver::session;
 import lib::llvm::llvm;
 import front::attr;
-import middle::trans;
 import middle::ty;
 import metadata::encoder;
+import middle::trans_common::crate_ctxt;
 import std::str;
 import std::fs;
 import std::ivec;
@@ -13,7 +13,6 @@ import option::some;
 import option::none;
 import std::sha1::sha1;
 import std::sort;
-import trans::crate_ctxt;
 import syntax::ast;
 import syntax::print::pprust;
 import lib::llvm::llvm::ModuleRef;

src/comp/metadata/encoder.rs

@@ -11,7 +11,7 @@ import std::ebmlivec;
 import std::map;
 import syntax::ast::*;
 import common::*;
-import middle::trans::crate_ctxt;
+import middle::trans_common::crate_ctxt;
 import middle::ty;
 import middle::ty::node_id_to_monotype;
 import front::attr;