Don't extern alloc unless needed to avoid requiring global allocators in #[no_std] environments

[udoprog]

Disabling the std feature causes extern crate alloc to be used, which in turn requires any downstream #[no_std] users to define a global allocator with a message like this:

error: no global memory allocator found but one is required; link to std or add `#[global_allocator]` to a static item that implements the GlobalAlloc trait

This change ensures that we only use extern crate alloc when it's actually needed, which is when the write feature is enabled.

Also note that std::string::String and std::vec::Vec are aliases for the corresponding alloc types, so importing it through std when it's available is not necessary.

It would arguably be cleaner for #![no_std] to always be enabled and add the following where std types and items are used, but changing this is up to you:

#[cfg(feature = "std")]
extern crate std;