channel.c: WIP remove redundant method check and added FUNC_ATTR_NONNULL_ALL macro

channel.c channel_defs.h helpers.c: added Error field to RequestEvent, added no_op handler func

channel.c: use const char* instead of string and cleanup

channel.c; channel_defs.h; helpers.c: removed error from event again; send errors directly to the channel without using handlers and events

channel.c: fixed memory leak and lint errors

api/private/dispatch.c; api/vim.c; msgpack_rpc/channel.c msgpack_rpc/helpers.c added Error* field to msgpack_get_handler_for; further refactored channel.c

channel.c:323 changed order of evaluation in if statement

channel.c: removed superflous whitespace

dispatch.c: review comment

This one behaves correctly eg in the presence of unicode

Fixes #8591

Ref #8474

doc: termios defaults. ref #6992
doc: :help shell-powershell
doc: provider: Python minimum version is 2.7, 3.4
doc: remove :!start special-case. #5844
doc: mention #7917 change which accepts empty Array for Dictionary parameter
doc: <Cmd> pseudokey
doc: lmap change #5658
doc: -s, -es

Move redundant common logic into a function.