input_dsl: expose InputMap layers; add From<SourceIter> for TokenIter

dsl/src/dsl_iter.rs

@@ -14,7 +14,10 @@ use crate::*;
 /// Provides a native [Iterator] API over the [ConstIntoIter] [SourceIter]
 /// [TokenIter::next] returns just the [Token] and mutates `self`,
 /// instead of returning an updated version of the struct as [SourceIter::next] does.
-#[derive(Copy, Clone, Debug, Default, PartialEq)] pub struct TokenIter<'a>(pub SourceIter<'a>);
+#[derive(Copy, Clone, Debug, Default, PartialEq)] pub struct TokenIter<'a>(
+    pub SourceIter<'a>
+);
+
 impl<'a> TokenIter<'a> {
     pub const fn new (source: &'a str) -> Self {
         Self(SourceIter::new(source))
@@ -31,6 +34,12 @@ impl<'a> Iterator for TokenIter<'a> {
     }
 }
 
+impl<'a> From<SourceIter<'a>> for TokenIter<'a> {
+    fn from (source: SourceIter<'a>) -> Self{
+        Self(source)
+    }
+}
+
 /// Owns a reference to the source text.
 /// [SourceIter::next] emits subsequent pairs of:
 ///   * a [Token] and