refactor(frontends/lean/local_context): do not use references in the local context
refactor(frontends/lean/elaborator): move placeholder_elaborator to its own module
refactor(frontends/lean/elaborator): move coercion_elaborator to its own module
refactor(library/data/list/basic): cleanup

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(emacs/lean-company): send dummy continuation when send wait
chore(emacs/lean-server): send signal info to lean-server-trace
feat(lean-server): handle signals (error/quit) in continuations

Related with #158
feat(emacs/lean-server): async task queue holds type of cmds as well