Fixes #843: Bind to embedded struct method or field

[matiasanaya]

This is an attempt to write test cases for the buggy codegen.builder#findBindTarget method as initially raised in #843 .

Some of the issues:

• It errors (and fails silently) if it finds an embedded exported interface within a struct.
• It won't look into methods from un-exported embedded structs.
• It can't bind to an interface.

I'm not entirely sure what's the expected behaviour in some of this cases since this docs, code comments here and examples (I'm certain I've seen an example on how to bind to an embedded interface) seem to contradict at times each other.

A way to fix some of these issues would be to use types.LookupFieldOrMethod. If there's some alignment on what the spec should be, I'm happy to update this PR with code that solves the issues.

The best way to run these tests is:

go generate ./codegen/testserver/embedded/... && go test ./codegen/testserver/embedded/... -count=1

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

Coverage decreased (-0.1%) to 61.957% when pulling a745dc7 on matiasanaya:fix/find-bind-target into f80cab0 on 99designs:master.

[vektah]

Looks good, PR description and title could use an update.

[matiasanaya]

@vektah thanks for the review. I've updated the PR title, fixed up the commits and cleaned up the unused code previously committed.

This PR fixes the following to issues:

• findBindTarget fails to bind to embedded structs if it finds an embedded exported interface within a struct.
• findBindTarget fails to bind to exported fields / methods of un-exported embedded structs.

I'm happy for this to be merged as is.

I'd love to do a followup PR to support embedded interfaces.