Add support for embedded structs

[duckbrain]

Closes #351

Currently, pop treats embedded structs as a distinct field (like the reflection package exposes). sqlx, which binds the values out of the struct treats the embedded fields as if on the parent (like the language makes appear to happen).

This checks for a newly introduced inline option to the db flag in the struct tags and inlines a struct (embedded or not), so that that struct's fields are included as if part of the parent object.

• db:",inline" or db:"-,inline" will inline the struct, but not include the field
• db:"name,inline"will include all of the fields on the struct, and the struct itself as the name field.
• db:"name"will include the struct as the name field. (Useful for struct types that implement Valuer and Scanner.)

This preserves the previous behavior by default, but allows embedded structs to be ignored as well.