Update findnext to use helper macros.

Author: kmsquire

base/bitarray.jl

@@ -1450,9 +1450,9 @@ end
 function findnext(B::BitArray, start::Integer)
     Bc = B.chunks
 
-    chunk_start = div(start-1, 64)+1
-    within_chunk_start = start-1-chunk_start*64
-    mask = _msk64-((1<<within_chunk_start)-1)
+    chunk_start = @_div64(start-1)+1
+    within_chunk_start = @_mod64(start-1)
+    mask = _msk64 << within_chunk_start
 
     if Bc[chunk_start] & mask != 0
         return (chunk_start-1) << 6 + trailing_zeros(Bc[chunk_start] & mask) + 1
@@ -1475,9 +1475,9 @@ function findnextnot(B::BitArray, start::Integer)
         return 0
     end
 
-    chunk_start = div(start-1, 64)+1
-    within_chunk_start = start-1-chunk_start*64
-    mask = (1<<within_chunk_start)-1
+    chunk_start = @_div64(start-1)+1
+    within_chunk_start = @_mod64(start-1)
+    mask = ~(_msk64 << within_chunk_start)
 
     if Bc[chunk_start] | mask != _msk64
         return (chunk_start-1) << 6 + trailing_ones(Bc[chunk_start] | mask) + 1