1.0.32.19: Support for :REPLACEMENT external-format / encoding modifier

1.0.32.19: Support for :REPLACEMENT external-format / encoding modifier

For a given keyword :foo naming an encoding, allow an external-format
(:foo :replacement <character>) such that any stream or octet coding
errors are automatically treated by using the <character> as replacement
instead.

To do this, wrap each of the functions in the external-format object
named by the keyword with a function establishing handlers for the
exceptional conditions. At the moment, the output restarts for c-string
external format conversion are not implemented (so handle specific condition
types like STREAM-FOOCODING-ERROR, not general FOOCODING-ERROR).