Object safety for traits

[tailhook]

Currently, rust complains:

error[E0038]: the trait `bytes::Buf` cannot be made into an object
  --> src/proto.rs:17:5
   |
17 |     buf: Option<Box<Buf>>,
   |     ^^^^^^^^^^^^^^^^^^^^^ the trait `bytes::Buf` cannot be made into an object
   |
   = note: method `get_u16` has generic type parameters
   = note: method `get_i16` has generic type parameters
   = note: method `get_u32` has generic type parameters
   = note: method `get_i32` has generic type parameters
   = note: method `get_u64` has generic type parameters
   = note: method `get_i64` has generic type parameters
   = note: method `get_uint` has generic type parameters
   = note: method `get_int` has generic type parameters
   = note: method `get_f32` has generic type parameters
   = note: method `get_f64` has generic type parameters

While I'm not sure if boxing buf is good for performance reasons, I'd prefer that boxing be allowed for flexibility.

This can be done in in two ways:

1. Use different methods get_u16_le and get_u16_be
2. Making a wrapper type: buf.little_endian_reader().get_u16()
   While slightly less convenient, I still think it's beneficial to make a trait object-safe.

This can only be done as a breaking change in 0.5 though.

[carllerche]

One option is to add where clauses to all these fns, but then they would not be available as part of the trait object.

Adding _be and _le suffixes to the trait is the better option here.

[tailhook]

Is adding where clauses a backward-compatible change? I.e. can we add where clauses deprecate the methods and add new ones with suffixes?

[carllerche]

I... think? But @alexcrichton would know better.