Introduce Signature type to avoid string comparisons.

Method resolution currently creates strings to then compare with strings formed
from methods in other dex files. The temporary strings are purely created for
the sake of comparisons. This change creates a new Signature type that
represents a method signature but not as a string. This type supports
comparisons and so can be used when searching for methods in resolution.

With this change malloc is no longer the hottest method during dex2oat (now its
memset) and allocations during verification have been reduced. The verifier is
commonly what is populating the dex cache for methods and fields not declared
in the dex file itself.

Change-Id: I5ef0542823fbcae868aaa4a2457e8da7df0e9dae
diff --git a/runtime/dex_file-inl.h b/runtime/dex_file-inl.h
index 2ee9244..c57a1e7 100644
--- a/runtime/dex_file-inl.h
+++ b/runtime/dex_file-inl.h
@@ -47,6 +47,10 @@
   return StringPiece(data, static_cast<int>(length));
 }
 
+inline const Signature DexFile::GetMethodSignature(const MethodId& method_id) const {
+  return Signature(this, GetProtoId(method_id.proto_idx_));
+}
+
 inline const DexFile::TryItem* DexFile::GetTryItems(const CodeItem& code_item, uint32_t offset) {
   const uint16_t* insns_end_ = &code_item.insns_[code_item.insns_size_in_code_units_];
   return reinterpret_cast<const TryItem*>