Add an `assaybind()` method

[multimeric]

There are a few use cases where you might want to be able to combine SummarizedExperiment "by assay", which can be thought of as combining along the Z axis in the summary diagram:

The most definitive way I've found to do this is to simply concatenate the assays: https://support.bioconductor.org/p/9136230/#9136235. Having an assaybind() method would not only streamline and simplify this approach, but would also allow us to add sensible handling of collisions in colData or rowData, and document what happens if the two assays don't have the same rows/columns (I'm not actually sure what this does).

I'm envisaging something like assaybind(...) -> se where ... and se are SummarisedExperiment. I guess it should be a generic as well, to support subclasses.

I'm happy to look into implementing this, also.