util: Cleanup CachedValue class.
util: Cleanup CachedValue class.

We use a Symbol for unassigned_value_sentinel and explictly assign
the compute_value property instead of using Object.assign. Both of
these are needed to type annotate it easily.

rht/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,rht/zulip,kou/zulip,kou/zulip,rht/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip