Simplify alloc/std imports

[FelixMcFelix]

Most files throughout OPTE have some form of the following import block:

cfg_if! {
    if #[cfg(all(not(feature = "std"), not(test)))] {
        use alloc::string::String;
        use alloc::string::ToString;
        use alloc::vec::Vec;
    } else {
        use std::string::String;
        use std::string::ToString;
        use std::vec::Vec;
    }
}

This is redundant, as these are re-exported identical types. This current construction stems from all our crates conditionally importing alloc or std – only std should be conditional. We should likely be doing the following:

Before	After
#![no_std] #[cfg(any(feature = "std", test))] #[macro_use] extern crate std; #[cfg(all(not(feature = "std"), not(test)))] #[macro_use] extern crate alloc;	#![no_std] #[cfg(any(feature = "std", test))] #[macro_use] extern crate std; #[macro_use] extern crate alloc;
cfg_if! { if #[cfg(all(not(feature = "std"), not(test)))] { use alloc::string::String; use alloc::string::ToString; use alloc::vec::Vec; } else { use std::string::String; use std::string::ToString; use std::vec::Vec; } }	use alloc::string::String; use alloc::string::ToString; use alloc::vec::Vec;