Add length function for strings

lib/Minilisp.pm

@@ -166,6 +166,12 @@ my %stdctx = (
   'string-not-greaterp' => sub { my ($a, $b) = @_; return !(lc($a) gt lc($b)); },
   'string-not-lessp' => sub { my ($a, $b) = @_; return !(lc($a) lt lc($b)); },
 
+  # string length
+  'length' => sub {
+    my ($a) = @_;
+    return length($a);
+  },
+
   # Bitwise operations
   'logand' => sub {
     my $v = -1;