update xfile. close #80.
add SWAP macro
support datum label reader
error on macroexpanding circular objects
update readme
Merge branch 'datum-label'
implement numerical I/O
Merge branch 'numerical-io'
fix bug on matching string litral
fix bug of matching symbol literal
<literal> superiors to <ellipsis>, when there is conflict
Merge remote-tracking branch 'upstream/master' into syntax-rules
remove definition of number->string