initial postbox for testing

not sure if we want to put this here yet, but it matches the design
and will be useful for testing

Fixes: https://github.com/damus-io/notedeck/issues/110
Suggested-by: Rob

src/draft.rs

@@ -1,8 +1,30 @@
+use std::collections::HashMap;
+
 #[derive(Default)]
 pub struct Draft {
     pub buffer: String,
 }
 
+#[derive(Default)]
+pub struct Drafts {
+    pub replies: HashMap<[u8; 32], Draft>,
+    pub compose: Draft,
+}
+
+pub enum DraftSource<'a> {
+    Compose,
+    Reply(&'a [u8; 32]), // note id
+}
+
+impl<'a> DraftSource<'a> {
+    pub fn draft(&self, drafts: &'a mut Drafts) -> &'a mut Draft {
+        match self {
+            DraftSource::Compose => &mut drafts.compose,
+            DraftSource::Reply(id) => drafts.replies.entry(**id).or_default(),
+        }
+    }
+}
+
 impl Draft {
     pub fn new() -> Self {
         Draft::default()