In the family of functions that create a text stream from a string (e.g.,
Streams::open_from_ISO_string, Str::new_from_ISO_string) we can mark the
input string as const since it is not supposed to be modified.
I ran into this when experimenting with trying to create a text stream
from the output of "getenv" which is "const char *". I ended up not
needing that, but this seems like a good change nonetheless.
