Deprecate base16-encoded strings usage in Geo.WKB

[wojtekmach]

Ref: #166 (comment)

There's a ton of deprecation warnings when running wkb_test.exs which I can't get to at the
moment. If someone would like to go ahead and fix those, that'd be very appreciated! Could be a
nice excuse to play with Sourceror [1] to write a script that does that. :)

[1] https://github.com/doorgan/sourceror

[bryanjos]

I'll take a look if I can get some time. And yes I'm so excited something like sourceror exists!