Improve prefix mapping mechanism

[milessabin]

The current PrefixMapping can only specialise mappings based on a path prefix. Whilst this is fully general, it can be extremely verbose to specify mappings using only this mechanism.

There are at least two scenarios which are not currently handled well,

1. the mapping should be determined by a path from an ancestor type. If this type appears in multiple locations in the schema, then the current model requires multiple prefix mappings to encode it.
2. the mapping resolves a whole subtree in the result, eg. by fetching data as Json from a web service via a nested EffectField. If descendent types below the top level of that subtree have mappings elsewhere (eg. SQL mappings for a different path from the root) then these other mappings will supercede the effect mapping for those descendents. This can be worked around by the use of additional prefix mappings, but again that can be extremely cumbersome if the descendent types are used in many other parts of the schema.

We need a more sophisticated mapping mechanism which can handle these scenarios smoothly.

[milessabin]

Fixed in #608.