Fix mmap_bitarray; extend it to Windows

Also add tests.
Fixes breakage introduced in d4d433d756
Update docs for previous change in ambiguous function warning.
Improve help()

* Discriminates between functions from different modules
  with the same name when calling help on the function itself
  (fix #4697)
* Attempts to prepend "Base." to function names
Merge pull request #4705 from JuliaLang/cb/bitmmapfix

Fix mmap_bitarray; make it available in Windows
Speed up (and test) sparse matrix-vector multiplication
test/bigint: fix failure on 32-bit system
Merge pull request #4711 from GlenHertz/patch-7

Update docs for previous change in ambiguous function warning message.
fix #4710, export/document searchindex and rsearchindex
improve printing of kw expressions
Merge pull request #4712 from JuliaLang/cb/helpfix

Another attempt at fixing #4697
fix #4718, Zip iterator was impure

this also makes it faster, and special-cases zipping 2 iterators
other iterators updated to be immutable
add eltype methods for iterators
a small inlining improvement that makes Zip2 faster
revert change to splitline benchmark. ref #4724
Merge pull request #4720 from JuliaLang/teh/sparsematvecmul

Speed up (and test) sparse matrix-vector multiplication
apply @timholy's optimizations to sparse * to a couple other cases
improve array intersect/union. fixes #4725
improve type of set union
quick hack to fix #4715
fix #4688, inlining too early inside method definitions