When the length to truncate is greater than the buffer's current
length, do nothing instead of clearing the contents.

This type is the counterpart to BufExt::take. It limits how many bytes
some `BufMut` can actually be filled with.

remove either feature

This brings `BytesMut` in line with `Vec<u8>` behavior.

This also fixes an existing bug in BytesMut::bytes_mut that exposes
invalid slices. The bug was recently introduced and was only on master
and never released to `crates.io`.

In order to fix a test, `BufMutExt::chain_mut` is provided. Withou this,
it is not possible to chain two `&mut [u8]`.

Closes #170

Rust versions bumped to the minimum needed for const slice len.

Makes the short-circuit checks inline-able, and the moves the actual
reserving code to an inner function.

- The return type of `BufMut::bytes_mut` is now
  `&mut [MaybeUninit<u8>]`.
- The argument type of `BufMut::bytes_vectored_mut` is now
  `&mut [bytes::buf::IoSliceMut]`.
- `bytes::buf::IoSliceMut` is a `repr(transparent)` wrapper around an
  `std::io::IoSliceMut`, but does not expose the inner bytes with a safe
  API, since they might be uninitialized.
- `BufMut::bytesMut` and `BufMut::bytes_vectored_mut` are no longer
  `unsafe fn`, since the types encapsulate the unsafety instead.

A `&str` cannot arbitrarily advance bytes, since it will panic if
advanced to the middle of a Unicode segment.

Bytes is a useful tool for managing multiple slices into the same region
of memory, and the other things it used to have been removed to reduce
complexity. The exact strategy for managing the multiple references is
no longer hard-coded, but instead backing by a customizable vtable.

- Removed ability to mutate the underlying memory from the `Bytes` type.
- Removed the "inline" (SBO) mechanism in `Bytes`. The reduces a large
  amount of complexity, and improves performance when accessing the
  slice of bytes, since a branch is no longer needed to check if the
  data is inline.
- Removed `Bytes` knowledge of `BytesMut` (`BytesMut` may grow that
  knowledge back at a future point.)

Verify no-std build and all features build

To make the library work as `no_std` we add an `std` feature which
is on by default. When it is off, we compile as `no_std` and make
parts of the API that require `std::io` conditional on the `std`
feature.

As consequence Buf::collect is removed as well, which is replaced with `Buf::into_bytes`. The advantage of `Buf::into_bytes` is that it can be optimized in cases where converting a `T: Buf` into a `Bytes` instance is efficient.

- Renames `bytes_vec` to `bytes_vectored` and `bytes_vec_mut` to
  `bytes_vectored_mut`.