Added quasiquoting

Also:
1) added `atom?` function to scm2secd.scm;
2) `compile` is now `secd-compile`;
3) naive implementation of quasiquoting; unquote-splicing not
implemented yet:
 - lexer and parser support for (quasi/un)quoting;
4) added some asserts to secd_ap();
5) `secdf_append` now takes 0,1,2,3... arguments;