- Add `Char` replication functions `prependChars` and `appendChars`
- Add padding functions `justifyLeft`, `justifyRight` and `center`.
- Move low-level `MArray` manipulation to their own module
  `Data.Text.Builder.Linear.Array`.
- Add corresponding tests and benchmarks.

Some situations may require a fresh empty buffer. For example, if one
want to define a function that append padded hexadecimal numbers, the
use of a mere `justifyRight` requires the input `Buffer` to be empty.

We introduce `newEmptyBuffer` to solve this issue. It has the caveat to
require an existing `Buffer`, because we need to to know if the array is
pinned or not.

Working example for padded hexadecimal numbers:

    -- | Convenient function to append padded hex
    infixl 6 |>&&
    (|>&&) ∷ (Integral a, FiniteBits a) ⇒ Buffer ⊸ (Word, a) → Buffer
    b |>&& (w, n) = case newEmptyBuffer b of
    (# b', empty #) → (b' |># "0x"#) >< justifyRight w '0' (empty |>& n)

    -- Example
    runBuffer \b → (b |># "Foo "#) |>&& (4, 0xf ∷ Word) |># "; "# |>&& (4, 0xabc ∷ Word)
    -- "Foo 0x000f; 0x0abc"