Implement sound map value aliasing

This implementation gets rid of the unsound `ShallowCopy` trait (#74 &
rust-lang/unsafe-code-guidelines#35 (comment)),
and replaces it with a wrapper type around aliased values.

The core mechanism is that the wrapper type holds a `MaybeUninit<T>`,
and aliases it by doing a `ptr::read` of the whole `MaybeUninit<T>` to
alias. It then takes care to only give out `&T`s, which is allowed to
alias. To drop, the implementation sets a thread-local that the wrapper
uses to determine if it should truly drop the inner `T` (i.e., to
indicate that this is the last copy, and the `T` is no longer aliased).

The removal of `ShallowCopy` makes some of the API nicer, and obviates
the need for `evmap-derive`. Unfortunately the introduction of the
wrapper also complicates certain bounds which now need to know about the
wrapping. Overall though, an ergonomic win.

The implementation passes all tests, and I _think_ it is sound (if you
think it's not, please let me know!). The one bit that I'm not sure
about is the thread-local if a user nests `evmap`s (since they'll share
that thread local).

Note that this does _not_ take care of #78.

Fixes #74.
Fixes #55 since `ShallowCopy` is no longer neeeded.

Author: jonhoo

Cargo.toml

@@ -1,2 +1,2 @@
 [workspace]
-members = ["evmap", "evmap-derive", "left-right"]
+members = ["evmap", "left-right"]

azure-pipelines.yml

@@ -12,18 +12,6 @@ stages:
         minrust: 1.40.0
         codecov_token: $(CODECOV_TOKEN_SECRET)
         dir: "evmap"
-    - job: derive
-      displayName: "Test evmap-derive"
-      pool:
-        vmImage: ubuntu-latest
-      steps:
-        - template: install-rust.yml@templates
-          parameters:
-            components:
-              - rust-src
-        - script: cargo test
-          displayName: cargo test
-          workingDirectory: "evmap-derive"
     - job: benchmark
       displayName: "Check that benchmark compiles"
       pool: