Remove `MemEncoder`.
It's only used in tests. Which is bad, because it means that
`FileEncoder` is used in the compiler but isn't used in tests!
`tests/opaque.rs` now tests encoding/decoding round-trips via file.
Because this is slower than memory, this commit also adjusts the
`u16`/`i16` tests so they are more like the `u32`/`i32` tests, i.e. they
don't test every possible value.
diff --git a/Cargo.lock b/Cargo.lock
index f6ee815..bff68df 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4059,6 +4059,7 @@
"indexmap",
"rustc_macros",
"smallvec",
+ "tempfile",
"thin-vec",
]
