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/ast.nim

@@ -47,6 +47,7 @@ type
       sliceSize*: Expr          ## nil for unsized slices T[]
     of tekOwn, tekPointer, tekRef, tekMutRef:
       pointerPointee*: TypeExpr
+      refLifetime*: string  ## only meaningful for tekRef/tekMutRef
     of tekDynRef:
       dynInterface*: string
     of tekTuple:
@@ -300,6 +301,7 @@ type
   TypeParam* = object
     name*: string
     bounds*: seq[string]   ## e.g. ["Comparable"] for <T: Comparable>
+    isLifetime*: bool      ## true for lifetime params like 'a
 
   # ---------------------------------------------------------------------------
   # Declarations