folly: adjust more headers to be -Wshadow-clean

folly: adjust more headers to be -Wshadow-clean

Summary:
This is part of what's required to make mcrouter compile warning-free
with gcc -Wshadow.  In case it's not obvious why this is worth doing,
see t2719164.

I've used two techniques:
rename one of the shadowed variables
bracket offending code with #pragma directives to disable the warning there.

* folly/Bits.h (BitIterator): Guard this function with #pragma
to avoid a warning about its member-shadowing "bitOffset" parameter.
* folly/Memory.h (StlAllocator): Rename parameter in trivial, one-line
function definition, s/alloc/a/, to avoid shadowing the member function.
Let me know if you'd prefer #pragma directives instead.
* folly/io/Cursor.h (pull,skip,clone): Rename parameter, s/length/len/,
not to shadow the member function name.

Test Plan:
build and run tests of a few tools that use these headers

Reviewed By: jon.coens@fb.com

FB internal diff: D940493