Make readuntil() raise IncompleteReadError if window size is exceeded

This commit makes readuntil() raise an IncompleteReadError if the
receive window fills up before a delimiter match is found, making this
case operate similar to when EOF is received or a signal is received on
the channel. In the case of readline(), this will cause a partial line
to be returned without a trailing '\n'. For direct calls to readuntil(),
the exception is raised and the caller can retrieve the partial data
from the "partial" field of the exception.

To support longer lines, a caller is free to call readline() or
readuntil() again in these cases, appending the data returned to the
previous partial data, repeating this process as many times as they
like. However, since the default window size is 2 MBytes, it's very
unlikely that this will be needed.