Start working on this again

* eglot.el (url-util): Require it.
(eglot--process-sentinel): pending continuations now are quads (added env).
(eglot--process-filter): Unwind message markers correctly if handling fails.
(eglot--obj): Simple macro.
(eglot--log-event): Add some info to logged event.
(eglot--environment-vars, eglot--environment): Helper vars.
(eglot--process-receive): Improve.
(eglot--process-send): Niver log.
(eglot--request): Use eglot--obj. Add environment.
(eglot--notify): New helper.
(eglot--protocol-initialize): RLS must like file://
(eglot--current-flymake-report-fn): New var.
(eglot--textDocument/publishDiagnostics): Use flymake from Emacs 26.
(eglot-mode): Proper minor mode.
(eglot--recent-changes, eglot--versioned-identifier): New stuff.
(eglot--current-buffer-versioned-identifier)
(eglot--current-buffer-VersionedTextDocumentIdentifier)
(eglot--current-buffer-TextDocumentItem, eglot--after-change)
(eglot--signalDidOpen, eglot--maybe-signal-didChange): New stuff.
(eglot-flymake-backend): More or less a flymake backend function.

Fix mode line

* eglot.el (mode-line-misc-info): conditionalize to eglot-mode

Fix some byte-compilation warnings

Overhaul async mechanism safety

Simplify `eglot--protocol-initialize`

* eglot.el (eglot--protocol-initialize): Simplify

Experimental diagnostic overlays

Minor cleanup

Cancel timeouts when process dies unexpectedly

Organize a bit

Improve `eglot--current-process'

Handle notifications

Introduce and use `eglot--current-process-or-lose'

Add a mode-line construct and some minor fanciness

Rename eglot--continuations eglot--pending-continuations

Remove a couple of comments

Initial commit