Linker code cleanups

Turn `Linker::Link()` into free functions

  As very little information was kept in the Linker class, we can get rid
  of the whole class and have the `Link()` as free functions instead; the
  environment target as well as the consumer are passed along through an
  `spv_context` object.
  The resulting linked_binary is passed as a pointer rather than a
  reference to follow the Google C++ Style guidelines.

  Addresses remaining comments from
  https://github.com/KhronosGroup/SPIRV-Tools/pull/693 about the SPIR-V
  linker.

Fix variable naming in the linker

  Some of the variables were using mixed case, which did not follow the
  Google C++ Style guidelines.

Linker: Use EXPECT_EQ when possible and update some test

* Replace occurrences of ASSERT_EQ by EXPECT_EQ when possible;
* Reformulated some of the error messages;
* Added the symbol name in the error message when there is a type or
  decoration mismatch between the imported and exported declarations.

Opt: List all duplicates removed by RemoveDuplicatePass in the header

Opt: Make the const version of GetLabelInst() return a pointer

  For consistency with the non-const version, as well as other similar
  functions.

Opt: Rename function_end to EndInst()

  As pointed out by dneto0 the previous name was quite confusing and could
  be mistaken with a function returning an end iterator.
  Also change the return type of the const version to a pointer rather
  than a reference, for consistency.

Opt: Add performance comment to RemoveDuplicateTypes and decorations

  This comment was requested during the review of
  https://github.com/KhronosGroup/SPIRV-Tools/pull/693.

Opt: Add comments and fix variable naming in RemoveDuplicatePass

* Add missing comments to private functions;
* Rename variables that were using mixed case;
* Add TODO for moving AreTypesEqual out.

Linker: Remove commented out code and add TODOs

Linker: Merged together strings that were too much splitted

Implement a C++ RAII wrapper around spv_context
diff --git a/include/spirv-tools/optimizer.hpp b/include/spirv-tools/optimizer.hpp
index 2141bda..b909648 100644
--- a/include/spirv-tools/optimizer.hpp
+++ b/include/spirv-tools/optimizer.hpp
@@ -410,7 +410,12 @@
 // The pass remaps result ids to a compact and gapless range starting from %1.
 Optimizer::PassToken CreateCompactIdsPass();
 
-// Creates a remove duplicate capabilities pass.
+// Creates a remove duplicate pass.
+// This pass removes various duplicates:
+// * duplicate capabilities;
+// * duplicate extended instruction imports;
+// * duplicate types;
+// * duplicate decorations.
 Optimizer::PassToken CreateRemoveDuplicatesPass();
 
 // Creates a CFG cleanup pass.