Client/server communication now uses HMAC for auth

This is to prevent a convoluted exploit that can trigger remote code execution.
Error handler now also sets HMAC header.
Initial clang_complete like parameter completion
make it prettier with conceal on and smarter on ESC
Adopted internal parameter markers
Improves regex for parameter matching
Changed parameter delimiters
Reduce diagnostic when popup completion is visible
Discards overloads by CV qualification in popup. Put into preview window
Add custom delimiters option
Sets concealed delimiter colors to normal text
Replaces error prone ESC mapping with an InsertLeave autocmd
Revert "Replaces error prone ESC mapping with an InsertLeave autocmd"

This reverts commit 527aa7dfe89661093248f5d95e998a072334906c.
InsertLeave didn't work, back to ESC mapping

This feedkeys call doesn't mess with arrows in terminal
Add minimal fork docs
ESC mapping still fails with arrows on terminal
Fixes GIF
Enable ESC mapping for GUI at last
dirty fix for enabling argument completion just for c/c++
Member original_string_ removed, more descriptive Key() used instead