Implement [more] app header formatting

Also, refactor out a utility method to format a Collection. This method can currently
replace some of the formatting done in module headers - but the goal is eventually
to be able to replace the code in fmt_list as well, such that there is 'one true way'
to format collections.

compiler/fmt/src/lib.rs

@@ -2,6 +2,7 @@
 // See github.com/rtfeldman/roc/issues/800 for discussion of the large_enum_variant check.
 #![allow(clippy::large_enum_variant)]
 pub mod annotation;
+pub mod collection;
 pub mod def;
 pub mod expr;
 pub mod module;