Add the dive tag for slice, array, and map elements

[leocavalcante]

Summary

• dive applies the tags after it to each slice element, array element, or map value. Tags before it stay on the collection.
• A struct element uses the validator the generator already emits. []*Address with required,dive,required checks a non-empty slice, non-nil pointers, and Address fields. A slice of structs with only required does not walk the elements.
• Map values are validated. Map keys are not. keys and endkeys stay unsupported. Nested arrays, pointer mixes the type encoding cannot tell apart, and in/nin on a slice of pointers are rejected.

Fixes #7

Test plan

• go test on parser, analyzer, common, and codegenerator
• make unittests
• make endtoendtests covers a slice of structs, a map value, a slice that does not dive, and []*Address
• CI build is green