Rename nrelementsof into lengthof

lengthof determines the length (number of elements)
of the first dimension of an array.
Implement stringstream_t

The implemented strstream_t now replaces string_t
for reading a stream of bytes.

The functions skipbyte_string,skipbytes_string,tryskipbytes_string
are removed from string_t.

Implemented new functions for string_t:
substr_string, initsubstr_string, shrinkleft_string, shrinkright_string and

Module utf8.c now adds utf-8 support to stringstream_t instead of string_t.
Adapt instream_t to stringstream_t

The interface is also extended with the function skipuntil_instream().
Implement textpos_t

textpos_t stores the current position of the reading cursor
of a reader of a text file.