chore: fix clippy warnings on latest nightly rust (#78)

btw `#[cfg(...)]` was used implicitly and expanded always lead to `use
alloc::collections::BTreeMap as Map;`
the second `cfg(std)` just doesn't work bcz `featureS`

src/gas_metering/backend.rs

@@ -116,7 +116,7 @@ pub mod mutable_global {
 				cost.saturating_add(rules.instruction_cost(instruction).unwrap_or(u32::MAX).into())
 			});
 			// don't charge for the instructions used to fail when out of gas
-			let fail_cost = vec![
+			let fail_cost = [
 				Instruction::I64Const(-1i64),           // non-charged instruction
 				Instruction::SetGlobal(gas_global_idx), // non-charged instruction
 				Instruction::Unreachable,               // non-charged instruction

src/stack_limiter/thunk.rs

@@ -1,12 +1,8 @@
-#[cfg(not(features = "std"))]
+use alloc::{collections::BTreeMap as Map, vec::Vec};
-use alloc::collections::BTreeMap as Map;
-use alloc::vec::Vec;
 use parity_wasm::{
 	builder,
 	elements::{self, FunctionType, Internal},
 };
-#[cfg(features = "std")]
-use std::collections::HashMap as Map;
 
 use super::{resolve_func_type, Context};