doughlang: Expressions in patterns

ast:
- Document operators
- Parameterize Pat with Annotation
- Consolidate Path and Lit into "Value" (Expr)
- Consolidate NamedRecord/Namedtuple into PatOp::TypePrefixed
- Allow Pat<Pat,*> patterns
- Additional helper functions on Expr and Pat

lexer:
- Support inner-doc comment syntax `//!`
  - Cleans up `///` or `//!` prefix

parser:
- Make Parse::Prec `Default`
- Allow expression elision after `..`/`..=`
- Fix Parser::consume not updating elide_do state
- Add token splitting, to make `&&Expr` and `&&Pat` easier
- error: Embed Pat precedence in ParseError

samples/elf.cl

@@ -0,0 +1,12 @@
+use num::{
+    i32, u32, u16, u8
+}
+
+type Elf32_Addr, Elf32_Half, Elf32_Off, Elf32_Sword, Elf32_Word, UChar = u32, u16, u32, i32, u32, u8;
+
+let <T> Option<T> = type Some(T) | None;
+// is equivalent to
+enum Option<T> {
+    Some(T),
+    None
+};