Merge #1504

1504: Simplify LSP handlers r=matklad a=kjeremy

Takes advantage of protocol inheritance via composition and simplifies some responses via the `From`/`Into` traits.

Co-authored-by: Jeremy Kolb <kjeremy@gmail.com>

diff --cc Cargo.lock
index 967018094378cdb7679e63080c5da2d5c08a89a5,a1bb9653559d93344b972fb2712c63cd199a0b32..3049ab3d9a66be29e18a51d4296adff314880935
--- 1/Cargo.lock
--- 2/Cargo.lock
+++ b/Cargo.lock
@@@ -2201,10 -2201,10 +2201,10 @@@ source = "registry+https://github.com/r
  "checksum linked-hash-map 0.5.2 (registry+https://github.com/rust-lang/crates.io-index)" = "ae91b68aebc4ddb91978b11a1b02ddd8602a05ec19002801c5666000e05e0f83"
  "checksum lock_api 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ed946d4529956a20f2d63ebe1b69996d5a2137c91913fe3ebbeff957f5bca7ff"
  "checksum log 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)" = "c84ec4b527950aa83a329754b01dbe3f58361d1c5efacd1f6d68c494d08a17c6"
- "checksum lsp-types 0.58.1 (registry+https://github.com/rust-lang/crates.io-index)" = "42c7edac51b6739544453fd33c7ae5e910d1ec4f2d9904cf1990f8f6581e26f5"
+ "checksum lsp-types 0.59.0 (registry+https://github.com/rust-lang/crates.io-index)" = "8961cde7f6f856771910edf37ff0729e59e02d2a8592a94c27495b746320e6ed"
  "checksum maplit 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)" = "08cbb6b4fef96b6d77bfc40ec491b1690c779e77b05cd9f07f787ed376fd4c43"
  "checksum matches 0.1.8 (registry+https://github.com/rust-lang/crates.io-index)" = "7ffc5c5338469d4d3ea17d269fa8ea3512ad247247c30bd2df69e68309ed0a08"
 -"checksum memchr 2.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "2efc7bc57c883d4a4d6e3246905283d8dae951bb3bd32f49d6ef297f546e1c39"
 +"checksum memchr 2.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "88579771288728879b57485cc7d6b07d648c9f0141eb955f8ab7f9d45394468e"
  "checksum memoffset 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "0f9dc261e2b62d7a622bf416ea3c5245cdd5d9a7fcc428c0d06804dfce1775b3"
  "checksum mio 0.6.19 (registry+https://github.com/rust-lang/crates.io-index)" = "83f51996a3ed004ef184e16818edc51fadffe8e7ca68be67f9dee67d84d0ff23"
  "checksum mio-extras 2.0.5 (registry+https://github.com/rust-lang/crates.io-index)" = "46e73a04c2fa6250b8d802134d56d554a9ec2922bf977777c805ea5def61ce40"