Make events per-parser

This makes the following changes:

1. Events in background threads are executed in those threads, instead of
being silently dropped

2. Blocked events are now per-parser instead of global

3. Events are posted in builtin_set instead of within the environment stack

The last one means that we no longer support event handlers for implicit
sets like (example) argv. Instead only the `set` builtin (and also `cd`)
post variable-change events.

Events from universal variable changes are still not fully rationalized.

src/builtin_cd.cpp

@@ -87,6 +87,10 @@ int builtin_cd(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
         return STATUS_CMD_ERROR;
     }
 
-    parser.vars().set_one(L"PWD", ENV_EXPORT | ENV_GLOBAL, std::move(norm_dir));
+    std::vector<event_t> evts;
+    parser.vars().set_one(L"PWD", ENV_EXPORT | ENV_GLOBAL, std::move(norm_dir), &evts);
+    for (const auto &evt : evts) {
+        event_fire(parser, evt);
+    }
     return STATUS_CMD_OK;
 }