Disallow promoting real-valued inputs to complex dtypes when values are outside of their real domain

[kgryte]

Initially discussed in #514 (comment).

In summary, when a user provides a real-valued input to a function such as log or sqrt and one or more values are outside of their real domain, implementations should not automatically promote to a complex floating-point dtype.

The specification does not support value based promotion, but we should reiterate this with particular regard to real-complex. Responsibility for type promotion should be punted to userland.