`ZeroCopyBuf` was convenient to use, but sometimes it did hide details
that some copies were necessary in certain cases. Also it made it way to easy
for the caller to pass around and convert into different values. This commit
splits `ZeroCopyBuf` into `JsBuffer` (an array buffer coming from V8) and
`ToJsBuffer` (a Rust buffer that will be converted into a V8 array buffer).
As a result some magical conversions were removed (they were never used)
limiting the API surface and preparing for changes in #19534.
Stop allowing clippy::derive-partial-eq-without-eq and fix warnings
about deriving PartialEq without also deriving Eq.
In one case I removed the PartialEq because it a) wasn't necessary,
and b) sketchy because it was comparing floating point numbers.
IMO, that's a good argument for enforcing the lint rule, because it
would most likely have been caught during review if it had been enabled.
This commit cleans up the Rust side of `import_key` by using a bunch of
enums instead of structs with "type" and "data" fields.
This commit does add some duplicated code for the time being, because
a lot of the other ops still need to get the same cleanup treatment.
