`PureByteSource` instance for Builders.

[piyush-kurur]

Starting from version 0.10.0.0, the Builder type is available
together with the bytestring library. A PureByteSource for builders
would be great.

• Points to check, is there a (stable) way to write out the contents
  of the builder to a given buffer?

Some description of the problem.

Builder got into the bytestring library from the Blazebuilder world
and is very fast to build lazy bytestring. Lazy bytestrings are essentially
a list of chunks each of which are strict bytestrings. The Builder infrastructure
makes sure that each chunk is reasonably large (fits in the l1 cache) instead of
making a lot of small chunks.

Of course a trivial way to have a PureByteSource instance is to
convert to Lazy ByteString. When computing a hash or encrypting using
a cipher, I believe there should be an internal way to spill the data
directly into a buffer and process it. This should be faster.