Fix memcpy/memset deprecation warning

[topikettunen]

Fix deprecation warnings mentioned in #220

Closes #220

• Deprecation warning for memset and memcpy with bytestring >= 0.11.5 #220

[topikettunen]

@andreasabel, Seems that fillBytes was introduced in GHC 7.10.1 and base version 4.8.0.0. Are we still interested in supporting even older versions that failed the check? If so, I guess we could yank the fillBytes function from the newer versions and just wrap it in some preprocessor.

[andreasabel]

@topikettunen, thanks for investigating!
There are basically 3 ways to proceed.

1. Just leave the deprecation warnings in and worry about them once the functions get actually removed.
2. Use base-compat to get fillBytes for older base: https://hackage.haskell.org/package/base-compat-0.13.0/docs/Foreign-Marshal-Utils-Compat.html#v:fillBytes
3. Drop GHC < 7.10.

Maybe @bgamari should decide the course of action here.

[andreasabel]

Rebased onto latest master.
Since we dropped testing with GHC 7, I see no obstacle in merging this now...