Fix #3692: Generalize implicit function subtyping.

[odersky]

With this change,

(implicit x: C): D  <:  (x: C): D

but not the other way around. This affects subtyping
of dependent implicit function types. Now:

(implicit x: C) => D  <:  (x: C) => D

See also #2000.

As a second change, prevent crashing on type mismatch errors
involving dependent implicit function types.

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