libsolidity/codegen: Use calldatacopy to cheaply zero memory instead of

codecopy.

Motiviation:
  Zero'ing memory is commonplace in contracts, but with the upcoming
  Layer-2 EVM translation layers and other on-chain verification
  mechanisms, using `codecopy` becomes a `costly` operation in those
  sandboxes. Using `calldatacopy` achieves the same thing, gas costs
  are also the same as codecopy, and is significantly cheaper in the `sandbox` situation.

libsolidity/codegen/CompilerUtils.cpp

@@ -605,7 +605,7 @@ void CompilerUtils::zeroInitialiseMemoryArray(ArrayType const& _type)
 		Whiskers templ(R"({
 			let size := mul(length, <element_size>)
 			// cheap way of zero-initializing a memory range
-			codecopy(memptr, codesize(), size)
+			calldatacopy(memptr, calldatasize(), size)
 			memptr := add(memptr, size)
 		})");
 		templ("element_size", to_string(_type.memoryStride()));