feat(library/match): add basic match_plugin that just invokes whnf before failing

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/match): add 'local' backtracking

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/unifier): better error message for invalid local context

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(frontends/lean/elaborator): local context

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/elaborator): use full local context for metavariables due to coercions and overloads

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/unifier): bug exposed by recent changes

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>