[Tolk] Get rid of ~tilda with mutate and self methods

This is a very big change.
If FunC has `.methods()` and `~methods()`, Tolk has only dot,
one and only way to call a `.method()`.
A method may mutate an object, or may not.
It's a behavioral and semantic difference from FunC.

- `cs.loadInt(32)` modifies a slice and returns an integer
- `b.storeInt(x, 32)` modifies a builder
- `b = b.storeInt()` also works, since it not only modifies, but returns
- chained methods also work, they return `self`
- everything works exactly as expected, similar to JS
- no runtime overhead, exactly same Fift instructions
- custom methods are created with ease
- tilda `~` does not exist in Tolk at all

tolk/lexer.h

@@ -38,6 +38,8 @@ enum TokenType {
   tok_var,
   tok_val,
   tok_redef,
+  tok_mutate,
+  tok_self,
 
   tok_annotation_at,
   tok_colon,
@@ -52,6 +54,7 @@ enum TokenType {
   tok_null,
 
   tok_identifier,
+  tok_dot,
 
   tok_plus,
   tok_minus,