support converting characters to integers

should be useful for reading from files/streams. incidentally, you can
now add characters together.
support converting integers to characters
print char escapes in -repr on strings
initial support for printing and reading utf8
fix str on characters

e.g. `(= (str \a \b \c \u269b) "abc⚛")`.
introduce helpers for encoding from and to utf8
support unicode in source files
fix -repr on characters outside the ascii range
add missing tests
Merge pull request #127 from heyLu/unicode-support

Unicode support and char improvments