Decode into pointer fields in-place.

It would be nice if decodePtr() would operate on the thing being pointed to rather than decoding into a new concrete value and assigning its address to the pointer.

I've found this behaviour handy using encoding/json, for example. Say I have a struct prepopulated with defaults. The struct corresponds to options nested under a key in a config file. I can create an instance of an anonymous struct with a field whose name refers to that key and value is a pointer to my original struct. I can then decode into the anonymous struct instance, and only fields present in the config file data will be updated, leaving the default fields intact.

Start by locating the decodePtr() entry point and reading how it currently handles an existing pointer. Trace its callers to understand the expected decoding path. Done means decoding updates the value already referenced by the pointer, preserving fields that are absent from the input instead of replacing the pointer with a new value.

Written by the indexing model from the issue text.