add functions:
  #starts_with
  -- ex.
  ```rust
    let buf: BFRDYN = "some text".into();

    let result = buf.starts_with("som");
    assert(result);
  ```
  #ends_with
  -- ex.
  ```rust
    let buf: BFRDYN = "some text".into();

    let result = buf.ends_with("ext");
    assert(result);
  ```
  #pop
  -- ex.
  ```rust
    let buf: BFRDYN = "some text".into();

    let last = buf.pop();
    assert('t', last);
  ```
  #popn
  -- ex.
  ```rust
    let buf: BFRDYN = "some text".into();

    let last = buf.popn(4);
    assert("text", last);
  ```
  #poph - pop head
  -- ex.
  ```rust
    let buf: BFRDYN = "some text".into();

    let head = buf.poph(4);
    assert("some", head);
  ```