please allow disabling implicit conversions in non-single-file use

[maddanio]

Hello,

I recently had to disable implicit conversions as they finally completely broke for us: If one of the types that json converts to are output streamable then streaming out json will immediately become ambiguous. Maybe that is further incentive for deprecating/disablong this and/or defaulting to disabled implicit conversions.

The problem is the disabling of implicit conversions seemingly only has been done for the single file header, but we really like to use json_fwd to speed up compilation, which is not possible with single file. Is it possible to enable those for non-single-file?