Phase 8.2.4: Lifetime syntax ('a) support

- Lexer tokenizes 'a as tkLifetime
- Parser accepts lifetimes in type params <'a> and reference types &'a T / &'a mut T
- AST TypeExpr has refLifetime field for tekRef/tekMutRef
- TypeParam gains isLifetime flag for generic monomorphization
- Sema infers lifetime params as no-op dummy values; unwraps pointee types for params inside refs
- HIR lower skips lifetime params when generating mangled names and substitution tables
- &mut expr syntax supported in parser (consumed as part of unary &)

src/token.nim

@@ -58,6 +58,7 @@ type
     tkStaticAssert    # static_assert
     tkComptime        # comptime
     tkDyn             # dyn
+    tkLifetime        # 'a (lifetime parameter)
 
     ##Punctuation
     tkLParen          # (
@@ -265,6 +266,7 @@ proc tokenKindName*(kind: TokenKind): string =
   of tkStaticAssert: "'static_assert'"
   of tkComptime: "'comptime'"
   of tkDyn: "'dyn'"
+  of tkLifetime: "lifetime"
   of tkLParen: "'('"
   of tkRParen: "')'"
   of tkLBrace: "'{'"