Multiple soundness issues

Calls std::str::from_utf8_unchecked() without any checks

The function xcb::xproto::GetAtomNameReply::name() calls
std::str::from_utf8_unchecked() on the raw bytes that were received from the
X11 server without any validity checks. The X11 server only prevents interior
null bytes, but otherwise allows any X11 client to create an atom for arbitrary
bytes.

This issue is tracked here: https://github.com/rust-x-bindings/rust-xcb/issues/96

xcb::xproto::GetPropertyReply::value() allows arbitrary return types

The function xcb::xproto::GetPropertyReply::value() returns a slice of type
T where T is an unconstrained type parameter. The raw bytes received from
the X11 server are interpreted as the requested type.

The users of the xcb crate are advised to only call this function with the
intended types. These are u8, u16, and u32.

This issue is tracked here: https://github.com/rust-x-bindings/rust-xcb/issues/95

Out of bounds read in xcb::xproto::change_property()

xcb::xproto::change_property has (among others) the arguments format: u8 and
data: &[T]. The intended use is one of the following cases:

• format = 8 and T = u8
• format = 16 and T = u16
• format = 32 and T = u32
  However, this constraint is not enforced. For example, it is possible to call
  the function with format = 32 and T = u8. In this case, a read beyond the
  end of the data slice is performed and the bytes are sent to the X11 server.

The users of the xcb crate are advised to only call this function with one of
the intended argument combinations.

This issue is tracked here: https://github.com/rust-x-bindings/rust-xcb/issues/94

‘Safe’ wrapper around std::mem::transmute()

The function xcb::base::cast_event() takes a reference to a
xcb::base::GenericEvent and returns a reference to an arbitrary type, as
requested by the caller (or found via type interference). The function is
implemented as a direct call to std::mem::transmute(). Since the return type
is not constrained, this allows transmution to an incorrect type or a type that
is larger than the X11 event that was passed in.

X11 events are mostly always 32 bytes large and this function works as intended.

Users are advised to only cast to the event structs provided by the xcb crate
(and hope for the best).

This issue is tracked here: https://github.com/rust-x-bindings/rust-xcb/issues/78