warning: use of `offset` with a literal
       --> src/buffer.rs:163:33
        |
    163 |         unsafe { Cursor::create(self.ptr.offset(1), self.scope) }
        |                                 ^^^^^^^^^^^^^^^^^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_offset_by_literal
        = note: `-W clippy::ptr-offset-by-literal` implied by `-W clippy::pedantic`
        = help: to override `-W clippy::pedantic` add `#[allow(clippy::ptr_offset_by_literal)]`
    help: use `add` instead
        |
    163 -         unsafe { Cursor::create(self.ptr.offset(1), self.scope) }
    163 +         unsafe { Cursor::create(self.ptr.add(1), self.scope) }
        |

    warning: use of `offset` with a literal
       --> src/buffer.rs:342:33
        |
    342 |             self.ptr = unsafe { self.ptr.offset(-1) };
        |                                 ^^^^^^^^^^^^^^^^^^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_offset_by_literal
    help: use `sub` instead
        |
    342 -             self.ptr = unsafe { self.ptr.offset(-1) };
    342 +             self.ptr = unsafe { self.ptr.sub(1) };
        |

Fixes #1948

tests: Use smallvec and thin_vec from rustc_data_structures

rust-lang/miri#4793

Fix LitByte::new().