Apply backend.result_type to equal, exp, expm1, full, greater* and less*

[james77777778]

Followed by #18482

The corresponding unit tests have also been added.

The principles in my mind, which are derived from the previous discussion:

• match the type inference of JAX
• if the integer input becomes a floating point output, use backend.floatx()
• try to support all dtypes in all backends (even if it introduces some overhead for checking)

[fchollet]

LGTM -- thanks for the great contribution!