Add a way to reset MemWriter

[SiegeLord]

MemWriter recently lost its ability to seek due to speed concerns, which removed any way of resetting/truncating it (i.e. seeking backwards). Could this be added? A possible API/implementation would be:

// Does nothing if new_len is greater than current length
pub fn truncate(&mut self, new_len: uint) { self.buf.truncate(new_len); }

Alternatively, perhaps a new constructor could be added which would wrap an existing Vec which would allow me to do this:

let writer = MemWriter::new();
let mut vec = writer.unwrap();
vec.truncate(new_len);
let writer = MemWriter::wrap(vec);

@erickt, I recall talking to you about this on IRC, so you may have some thoughts.

[SiegeLord]

Alternatively, given how simple the MemWriter implementation is now, perhaps Writer should just be implemented on top of for Vec, thus obtaining this feature automatically.

[xitep]

I'd welcome the MemWriter::wrap approach. This would allow a wider range of possibilities of sharing the buffer. Though, MemWriter::truncate would be a nice to have.

[steveklabnik]

/cc @aturon

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#872