Fuse ops and save alloc in Fpf (prev FPFilter).

This replaces Fpfilter with a new tpye Fpf. This acts in a similar way,
but makes heavy use of macros to make it possible to completely avoid
allocations in the common case when doing sign tests on algebraic
expressions.
Rename Fpf => FpFilter.
Calculate ind at compile time when possible.
Move FpFilter to spire.math.
Add FpFilter benchmark.
Don't truncate BigDecimals without any context.
Add some property tests for orientation tests.
Merge remote-tracking branch 'upstream/master' into topic/fpfilter-macros

Conflicts:
	tests/src/test/scala/spire/algebra/RingTest.scala
Merge pull request #254 from tixxit/topic/fpfilter-macros

Floating Point Filter macro-fication