add support for begin in Sex

begin-wrapped blocks are now creating lexical scopes

tests/run.scm

@@ -21,7 +21,7 @@
 (test '%fun (atom-to-fmt-c 'fn))
 (test '%prototype (atom-to-fmt-c 'prototype))
 (test '%var (atom-to-fmt-c 'var))
-(test '%begin (atom-to-fmt-c 'begin))
+(test '%block-begin (atom-to-fmt-c 'begin))
 (test '%define (atom-to-fmt-c 'define))
 (test '%pointer (atom-to-fmt-c 'pointer))
 (test '%array (atom-to-fmt-c 'array))