* pkg/eval/str: move builtin ord and chr to str
Move builtin string function ord and chr to the str module and rename to
to str:to-codepoints and str:from-codepoints respectively as suggested
in #851.
* pkg/eval/str: add from-utf8-bytes & to-utf8-bytes
Add from-utf8-bytes and to-utf8-bytes functions to the str module. This
functions differ from their *-codepoints in that they handle utf8 bytes
instead of whole codepoints. Closes#851
* pkg/eval/str: range check for codepoint and bytes
str:from-codepoints
Add check if arguments codepoints are within valid unicode range, return
an OutOfRange error otherwise. Return a BadValue error if the codepoint
isn't valid. Add/change testcases.
str:from-utf8-bytes
Add check if byte arguments are within valid range, return an OutOfRange
error otherwise. Return a BadValue error if the byte sequence isn't a
valid UTF-8 sequence. Add/change testcases.
Add additional test if piping from str:to-codepoints/str:to-utf8-bytes
to str:from-codpoints/str:from-utf8-bytes returns the original input.
The implementations are mostly copied from the builtin joins, replaces and
splits, with small changes to the error behavior in the join function.
The versions in the builtin module are now deprecated.
