Don't drop a coercion in handling symbol projections

[lukemaurer]

When resolving a symbol projection to some simple s, we then
recursively try and resolve s as a symbol projection. If s carries a
coercion, we need to apply that coercion to the final result.

Unfortunately, I don't have a test case for this, because I'm not sure
it's currently possible to trip it: one would need a symbol that gets
set to a recursive occurrence, and yet that recursive occurrence needs
to be an alias to some other symbol projection. You'd want to do
something like

let foo = Sys.opaque_identity 42

let rec waste b = if b then waste false else bar [@@inline]
and bar = foo

but the and bar = foo part is not valid OCaml.

At any rate, I ran tests with the shortcut in
create_coerced_singleton_let removed, so the basic functionality of
wrapping a Named.t in a coercion is working.