[error] Implement std::error::Error on errors

While we're here, also relax `Dispaly for AlignmentError<Src, Dst>` to
permit `Dst: ?Sized` in exchange for `Dst: KnownLayout`. This is an
important relaxation since our APIs permit performing conversions into
unsized destination types with runtime alignment checking.

Also make all errors `Send + Sync` regardless of `Dst`, which only
exists at the type level, but is never instantiated.

Finally, make `Display`'s verbosity conditional on `debug_assertions`.

Makes progress on #1297

Co-authored-by: John Wrenn <jswrenn@amazon.com>

Cargo.toml

@@ -60,10 +60,11 @@ alloc = []
 derive = ["zerocopy-derive"]
 simd = []
 simd-nightly = ["simd"]
+std = ["alloc"]
 # This feature depends on all other features that work on the stable compiler.
 # We make no stability guarantees about this feature; it may be modified or
 # removed at any time.
-__internal_use_only_features_that_work_on_stable = ["alloc", "derive", "simd"]
+__internal_use_only_features_that_work_on_stable = ["alloc", "derive", "simd", "std"]
 
 [dependencies]
 zerocopy-derive = { version = "=0.8.0-alpha.14", path = "zerocopy-derive", optional = true }

README.md

@@ -82,6 +82,11 @@ for network parsing.
   the `alloc` crate is added as a dependency, and some allocation-related
   functionality is added.
 
+- **`std`**
+  By default, `zerocopy` is `no_std`. When the `std` feature is enabled, the
+  `std` crate is added as a dependency (ie, `no_std` is disabled), and
+  support for some `std` types is added. `std` implies `alloc`.
+
 - **`derive`**
   Provides derives for the core marker traits via the `zerocopy-derive`
   crate. These derives are re-exported from `zerocopy`, so it is not