spec/ebnf: macroexpand order, set!, letfn primitive, numeric tower

Bring the formal definition in line with this session's language work:
- grammar.ebnf: numbers are a real tower (exact integer / Ratio / double); the M
  suffix reads a real BigDecimal, N an exact integer (drop the stale Janet note).
- 02-reader S5: M is a real java.math.BigDecimal with scale-insensitive equality.
- 03-special-forms: document the read -> macroexpand -> analyze order (macros
  expand before special-form dispatch); special-form heads are not shadowable but
  macros are and value-position locals may be named like a special; set! on a var
  sets the innermost binding (else root); letfn is a primitive with letrec*
  semantics.

docs/spec/02-reader.md

@@ -43,7 +43,10 @@ exponent := [eE] ['+'|'-'] digits
 - S5. Trailing `N` (BigInt) and `M` (BigDecimal) suffixes are part of the
   grammar; their value semantics are the §4 numeric-tower question.
   Implementations without those towers SHOULD read them as the nearest
-  numeric type and MUST document the choice.
+  numeric type and MUST document the choice. The Chez host carries the full
+  tower: `N` reads as an exact integer (arbitrary precision) and `M` as a real
+  BigDecimal — `1.5M`, `0.0M`, `3M` — with value equality ignoring scale
+  (`1.0M = 1.00M`), `(class 1.5M)` ⇒ `java.math.BigDecimal`, and `decimal?` true.
 
 ### Symbols and keywords