Local `define-macro`

[jpellegrini]

@egallesio

STklos (like Gauche) only allows define-macro in top-level. Is there a strong reason why this is so in STklos?

For example, in Bigloo, Ypsilon, Kawa, Guile and Gambit it's OK:

(let ((x 10))
  (define-macro (a) 2)
  (+ (a) x))                         => 12

(In this case, Bigloo and Kawa create a global macro; the others don't)

Why don't we translate internal define-macro to %let-syntax?

(I'm asking because SRFI-247 for example would necessarily have to expand certain forms info internal defines)