Optimize encodeUtf8 by checking if the byte array is already pinned

The byte array underlying `Text` [might already be pinned](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/ffi.html#pinned-byte-arrays):

> A byte array [and by extension Text] can be pinned as a result of three possible causes:
>
> 1. It was allocated by newPinnedByteArray#. [Text is never constructed using this primitive]
> 2. It is large. Currently, GHC defines large object to be one that is at least as large as 80% of a 4KB block (i.e. at least 3277 bytes).
> 3. It has been copied into a compact region. The documentation for ghc-compact and compact describes this process.

Especially 2 seems likely and a case where we could save a significant amount of time by avoiding copying. Could we use [`isByteArrayPinned#`](https://hackage.haskell.org/package/base-4.14.1.0/docs/GHC-Exts.html#v:isByteArrayPinned-35-) to check if the byte array is already pinned and in that case avoid copying?

Start at encodeUtf8 and the Text byte-array representation, then read GHC's pinned-byte-array documentation and the isByteArrayPinned# reference. Done means avoiding the copy when the underlying byte array is already pinned while preserving UTF-8 encoding behavior.

Written by the indexing model from the issue text.