Clean up JSON-RPC types and deserialization code

[ebkalderon]

Added

• Implement std::str::FromStr for Request and Response.
• Implement From<jsonrpc::ErrorCode> for i64.

Changed

• Avoid heap allocation in Version deserialization.
• Make Version a private struct instead of pub(crate).
• Un-derive Copy for Version because it's unnecessary.
• Group jsonrpc::Result<T>and not_initialized_error() with all the error stuff, re-export it to parent module for public access
• Replace custom Serialize/Deserialize implementations for ErrorCode with #[serde(into/from)].
• Move Request/RequestBuilder to src/jsonrpc/request.rs submodule.
• Move Response to src/jsonrpc/response.rs submodule.
• Better document panic behavior for lsp-types conversions as Request::from_{notification,request} doc comments.

This pull request contains a grab-bag of miscellaneous improvements to the jsonrpc module that should improve deserialization performance slightly, but mostly make the code easier to read, grok, and maintain.

It was also supposed to include some major improvements to deserializing request params and ids, and handling invalid JSON-RPC messages in the Message type (see commits in the full improve-jsonrpc-deserialization branch), but I ran into an inconsistency in the official LSP spec and chose to not pursue this for now. I've asked for clarification in microsoft/language-server-protocol#1686, filed earlier today.

[ebkalderon]

Regarding microsoft/language-server-protocol#1686: It turns out that the params field of the telemetry/event notification not being of type object | array is a bug in the LSP spec. I will open a PR against lsp-types fixing this by changing the Params associated type for this notification to something like OneOf<serde_json::Map<String, Value>, Vec<Value>>.

This also means it should be safe to add the strict Params type from improve-jsonrpc-deserialization (4a0e471) after all! 🎉