Summary: `decodeVarint` now accepts all of `StringPiece`, `MutableStringPiece`, `ByteRange`, and `MutableByteRange`.

Test Plan: fbconfig -r folly unicorn/utils && fbmake runtests_opt -j32

Reviewed By: ott@fb.com, lucian@fb.com

Subscribers: trunkagent, chaoyc, search-fbcode-diffs@, unicorn-diffs@, folly-diffs@, yzhan, yfeldblum

FB internal diff: D1836805

Signature: t1:1836805:1423534085:cca5c3e83ad699e5d56e1d1e3394644ec3f94dab