Rename directory utf8cols -> utf8
want to put other utf8 tests there
utf8-tests use EXECENV, add readstring tests
Migrate these C tests to use .test.c
Delete files no longer needed - migrating c tests
(trivial) fix usage for compileline
Rewrite readstring

Now readstring has the following better properties:
 - fewer string copies
 - simpler
 - works correctly reading to the end of stdin
 - respects binary/text mode setting on the channel

In order to do that, I needed to make some minor modifications
to QIO to support reading until the end of the file as
a style setting for binary and text string reading
(qio_channel_read_string is the binary version and
 qio_channel_scan_string is the text version).