いくつかの文字関数を追加

[hidekuno]

• digit->integer

gosh> (digit->integer #\4)
4
gosh> (digit->integer #\e 16)
14
gosh> (digit->integer #\9 8)
#f
gosh> (digit->integer #\4 10)
4
gosh>

• integer->digit

gosh> (integer->digit 13 16)
#\d
gosh> (integer->digit 8 8)
#f
gosh> (integer->digit 7 8)
#\7
gosh> (integer->digit 9 10)
#\9
gosh>