Parsing support for snake_case identifiers

In this initial commit, I have done the following:

- Added unit tests to roc_parse's ident.rs file to cover at least the
  simplest Ident enum cases (Tag, OpaqueRef, and simple Access)
- Added '_' as a valid "rest" character in both uppercase and lowercase
  identifier parts
- Updated the test_syntax snapshots appropriately

There is still a lot left to do here. Such as:

- Do we want to allow multiple '_'s to parse successfully?
- Handle qualified access
- Handle accessor functions
- Handle record update functions
- Remove the UnderscoreInMiddle case from BadIdent
- Write unit tests for Malformed Idents

I am not a "Rustacean" by any means, but have been through the Book in
years past.  Any feedback on the way I wrote the tests or any other part
of the implementation would be very appreciated.

crates/compiler/fmt/src/collection.rs

@@ -35,6 +35,7 @@ impl Braces {
 
 pub fn fmt_collection<'a, 'buf, T: ExtractSpaces<'a> + Formattable + std::fmt::Debug>(
     buf: &mut Buf<'buf>,
+    flags: &crate::annotation::MigrationFlags,
     indent: u16,
     braces: Braces,
     items: Collection<'a, T>,
@@ -109,7 +110,7 @@ pub fn fmt_collection<'a, 'buf, T: ExtractSpaces<'a> + Formattable + std::fmt::D
             }
 
             buf.indent(item_indent);
-            item.item.format(buf, item_indent);
+            item.item.format(buf, flags, item_indent);
 
             buf.indent(item_indent);
             buf.push(',');
@@ -152,7 +153,7 @@ pub fn fmt_collection<'a, 'buf, T: ExtractSpaces<'a> + Formattable + std::fmt::D
                 buf.spaces(1);
             }
 
-            item.format(buf, indent);
+            item.format(buf, flags, indent);
             if iter.peek().is_some() {
                 buf.push(',');
             }