Update series syntax and binder scope handling

Refine the BNF for `series` to explicitly include the `lookback_limit`
and `schema`.
Introduce scope management within the `bind` function for `if/else`
branches to ensure correct context handling during compilation.
Add `Again` and `GetField` node kinds to the `Specializer`.
Improve lexer to ignore invisible characters within identifiers,
demonstrated by a new test case.

docs/BNF.md

@@ -92,7 +92,8 @@ This document defines the syntax (BNF) and semantics of the Myc language, a Lisp
 - **Vectors (Tuples):** Enclosed in square brackets `[1 2 3]`. Evaluates to a vector/tuple structure.
 - **Records:** Enclosed in curly braces with keyword keys and any expression as values `{:id 101 :name "Alice"}`. They provide O(1) field access using internal memory layouts. Structural equality `(= {:a 1} {:a 1})` is `true`.
 - **Series:** A core concept for financial analysis. Series are "infinite" queues with a maximum length (lookback). 
-  - They are created via the `(series ...)` function specifying a record layout (e.g., `(series {:price :float :volume :int})`). 
+  - They are created via the `(series lookback_limit schema)` function. The `schema` can be a record layout (e.g., `{:price :float}`) or a type keyword (e.g., `:float`).
+  - Example: `(series 100 {:price :float :volume :int})`.
   - **Indexing:** You access items in a series by calling it or an extracted field like a function with an integer index: `(my_series 0)`. **Crucially, index `0` represents the most recently pushed item.** Index `1` is the second most recent, and so on (lookback indexing).
 
 ## 4. Special Forms and Evaluation logic