Proposal: merge gojsonpointer and gojsonreference here

[ahmetb]

Turns out this project (which is great) depends on

• https://github.com/xeipuuv/gojsonpointer
• https://github.com/xeipuuv/gojsonreference

both of which have not been updated in a long time (meaning: stable enough) and small (meaning: they are single source file repos).

It just creates more hassle to do vendoring management for extra packages when they are almost exclusively used in gojsonschema.

Perhaps can we cut dependency to these extraneous repositories and just copy them locally under gojsonschema? @sigu-399 @xeipuuv what do you think?

[xeipuuv]

Sounds reasonable :)