Remove "using namespace" from header and move Instruction to dev::eth.

test/libsolidity/SolidityExpressionCompiler.cpp

@@ -33,6 +33,7 @@
 #include <test/Options.h>
 
 using namespace std;
+using namespace dev::eth;
 using namespace langutil;
 
 namespace dev

test/libsolidity/SolidityOptimizer.cpp

@@ -109,7 +109,7 @@ public:
 		bytes realCode = bytecodeSansMetadata(_bytecode);
 		BOOST_REQUIRE_MESSAGE(!realCode.empty(), "Invalid or missing metadata in bytecode.");
 		size_t instructions = 0;
-		solidity::eachInstruction(realCode, [&](Instruction _instr, u256 const&) {
+		dev::eth::eachInstruction(realCode, [&](Instruction _instr, u256 const&) {
 			if (!_which || *_which == _instr)
 				instructions++;
 		});