If the Statement argument only has on element, emit the element.
Forgot to support Function in Emitter#emit.

* Fixes a "stack level too deep" bug.
Moved Field#to_s logic into Emitter#emit_field.

* Made Field Emittable.
* Have Emitter#emit pass Symbols to #emit_keyword.
Added InjectionExpr, as a special context for and { } / or { }.
Patch version bump.
Added another SQLi example.
Allow emit_operator to emit multiple keywords.
Use /^\W+$/ instead of matching alphabetic Strings.
Added another SQLi example.
Added another SQLi example.
Added emit_argument, so that sub-Statements are wrapped in ( ).
Added another SQLi example.
Added Clauses#union_all.
Added another SQLi example.
Minor version bump due to the added functionality.
Added @api and @since tags.