Group XForm lookups in case.get_forms

This is a nice win.  There's a slow (custom) report using this call on every row.  In local testing it took around a half a second per call.  This change brings that to around 0.05s (varying per case, of course), which represents around a 10x speed-up.  The result is still wrapped, and it's a list (not iterator), so the API should be unchanged.
how does this deal with the case where there's a dangling form reference? (The case `ResourceNotFound` above was dealing with.)