Previously we refused to emit 'autoConvert_*' functions if any field was not
convertible.  The way around this was to write manual Conversion functions, but
to do so safely you must handle every fields.  Huge opportunity for errors.

This PR cleans up the filtering such that it only operates on types that should
be converted (remove a lot of code) and tracks when fields are skipped.  In
that case, it emits an 'autoConvert' function but not a public 'Convert'
function.  If there is no manual function, the compile will fail.

This also means that manual conversion functions can call autoConvert functions
and then "patch up" what they need.