Change initialization for implicit-conversion calls.

Change the unit tests so calls to non-explicit constructors use
copy-list-initialization when possible, instead of
direct-list-initialization everywhere.
Add conversion from higher-rank objects.

Add (explicit) constructor in both "complex_it" and "complex_rt" (both
cases) to convert from an object of the same type, but of a higher rank
and with any component type.

Also add a missing "constexpr."
Add conversion to objects with explicitly-convertible component types.

Add constructor to "complex_it" and "complex_rt" (both cases) that
explicitly converts from objects of any length with component types that
are explicitly convertible.  Explicitly mandate implicit conversion for
some previous constructors.

Fix an compiler-internal error in a previous constructor for
"complex_rt" (recursive case) introduced by the new code above.  Purge
the dependency on cross-philosophy conversion in the process.  Re-enable
the implementation that was blocked in commit 3789d15..., except the
refined problem areas.

Correct a spelling error.