Define the "mrow" algorithm.

[bfgeek]

As above.

This needs to define what the layout pass of the algorithm is, as well as the intrinsic sizes pass.

Specific care here needs to be taken with how child margins are handled (if at all).

[fred-wang]

Regarding intrinsic sizes, the mpadded element can be problematic: I opened #81

[fred-wang]

It's now described

https://mathml-refresh.github.io/mathml-core/#layout-of-mrow

but we need to fix #111

[fred-wang]

I think this can be closed now, we have spec details and tests