Implemented ToCStr for gstr::Utf8
Replaced linkage to strlen with use of alloc::libc_heap

Should do it like Rust core libs do and abort on OOM.
Removed a murky and obsolete comment

We have no glob use anymore, and I intend to suppress binding generation
for types that would be implemented specially by grust. So the whole
paragraph does not apply any longer.
Ramified Utf8 into Utf8Buf and Utf8Str

To address all possible performance/flexibility scenarios, offering
two types to represent a GLib-allocated null-terminated UTF-8 string.
The functionality of old Utf8 is spread between them.
Utf8Buf just wraps the pointer, can't tell length, does not work
as a slice. It can be copied out into a String or moved into an
Utf8Str. The latter has scanned the string for length, so it works
as a slice, collection, and so forth.
Both types implement ToStr's with_c_str* methods with zero copying.