Add some string functions

lib/Minilisp.pm

@@ -227,6 +227,14 @@ 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 operations
+  'string-upcase' => sub { return uc(shift); },
+  'string-downcase' => sub { return lc(shift); },
+  'string-capitalize' => sub {
+    my $str = shift;
+    return $str =~ s/(\S+)/ucfirst($1)/erg;
+  },
+
   # Bitwise operations
   'logand' => sub {
     my $v = -1;