Allow specifying the module of the imported 'gas' function (#140)

* No need for mem::replace when doing a simple assignment

* Allow specifying the module of the imported 'gas' function

This allows users to place the imported function inside a custom
module instead of the generic 'env' module.

cli/gas/main.rs

@@ -17,7 +17,7 @@ fn main() {
 	let module = parity_wasm::deserialize_file(&args[1]).expect("Module deserialization to succeed");
 
 	let result = utils::inject_gas_counter(
-		module, &Default::default()
+		module, &Default::default(), "env"
 	).expect("Failed to inject gas. Some forbidden opcodes?");
 
 	parity_wasm::serialize_to_file(&args[2], result).expect("Module serialization to succeed")