core: remove vendored lazy_static on no-std

[hawkw]

Motivation

Currently, no_std targets use a vendored version of lazy_static that
uses the spin crate's Once type, while the std target uses the
once_cell crate's Lazy type. This is unfortunate, as the
lazy_static macro has a different interface from the Lazy cell type.
This increases the amount of code that differs based on whether or not
std is enabled.

Solution

This branch removes the vendored lazy_static macro and replaces it
with a reimplementation of once_cell::sync::Lazy that uses
spin::Once rather than once_cell::sync::OnceCell as the inner "once
type". Now, all code can be written against a Lazy struct with the
same interface, regardless of whether or not std is enabled.

Note that this change is only against v0.1.x, since the vendored
lazy_static! macro has already been removed on master.

[notgull]

Doesn't the spin crate have its own implementation of Lazy?

[hawkw]

Doesn't the spin crate have its own implementation of Lazy?

It does in recent versions. The vendored version used in tracing predates the addition of that API. We could potentially just update the vendored code to a newer version of spin; this felt easier as it didn't involve deleting a bunch of unused code.