Add newEmptyBuffer

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"

src/Data/Text/Builder/Linear/Core.hs

@@ -18,6 +18,7 @@ module Data.Text.Builder.Linear.Core (
   lengthOfBuffer,
   dropBuffer,
   takeBuffer,
+  newEmptyBuffer,
 
   -- * Text concatenation
   appendBounded,
@@ -112,6 +113,16 @@ memptyPinned = runST $ do
   arr ← A.unsafeFreeze marr
   pure $ Text arr 0 0
 
+-- | Create an empty 'Buffer'.
+--
+-- The first 'Buffer' is the input and the second is a new empty 'Buffer'.
+--
+-- Note: a previous buffer is necessary in order to create an empty buffer with
+-- the same characteristics.
+newEmptyBuffer ∷ Buffer ⊸ (# Buffer, Buffer #)
+newEmptyBuffer (Buffer t@(Text arr _ _)) =
+  (# Buffer t, Buffer (if isPinned arr then memptyPinned else mempty) #)
+
 -- | Duplicate builder. Feel free to process results in parallel threads.
 -- Similar to
 -- [@Dupable@](https://hackage.haskell.org/package/linear-base/docs/Prelude-Linear.html#t:Dupable)