Improve CHECK_<op> logging.

This patch lets us show the lhs and rhs when a relational check fails. (I show
both, even though that looks silly in cases like CHECK_EQ(rc, 0) where we'll
say "rc=-1, 0=0", because it's helpful in cases like CHECK_LT(i, size()) where
we want "i=4, size=2".)

I had to add a few operator<<s for enums, and fix a bunch of signed/unsigned
mismatches, and put the StringPiece operator<< in the right namespace.

Change-Id: I390f38bd97b3f50e12182f36ff027ca067c48d69
diff --git a/src/assembler.cc b/src/assembler.cc
index d0a8aa4..689e0c8 100644
--- a/src/assembler.cc
+++ b/src/assembler.cc
@@ -61,7 +61,7 @@
 
   // Verify internal state.
   CHECK_EQ(Capacity(), kInitialBufferCapacity);
-  CHECK_EQ(Size(), 0);
+  CHECK_EQ(Size(), 0U);
 }