Default to getfield rather than getproperty for field access

[piever]

This makes StructArrays work by default on struct types that customize getproperty.

We should probably allow to customize this mechanism by overloading a StructArray function (say StructArray.getentry), which would need to be overloaded at the same time as staticschema and createinstance.

[piever]

Probably relevant to look into Tables.getcolumn for a similar interface.