Windows: download busybox-win32 during make win-extras

copy it to echo.exe and printf.exe during make dist
This allows test/spawn.jl to succeed on Windows, since echo and printf
are only included as shell scripts in msysgit (and an echo.bat in
contrib/windows), but libuv cannot spawn shell or bat scripts.
In-place filt!, allow passing initial filter state

This redefines `filt` as `filt!` with a few modifications to perform its
filtering in-place. `filt` is defined as a simple wrapper, copying the vectors
that `filt!` modifies.

Further, both methods now allow an optional argument for the initial filter
state vector. This vector is modified in-place in `filt!`.
Be smarter about the initial filter state

Compute an initial state for filt such that its response to a step function is
steady state.
Revert "Be smarter about the initial filter state"

This reverts commit 19ba34c001152977d4fc011c75e1e08bb260aacf.
Fix copy! for ranges
NEWS and doc for filt state and filt!
Add a 3-argument method for scale! for scalars

Just like as scale! allows a pre-allocated argument for scaling a matrix by a
vector, this allows specifying a third array as the output for scaling by
scalars.
Refactor filt! to use an output argument

- No longer modifies the filter state in place, instead uses a copy
Update filt! API to have out as its first argument

Add a secondary method with keyword arguments to allow si and the output array
to be optional.
deps/Makefile: reinstall-llvm target to reinstall LLVM
deps/Makefile: allow soft override of LLVM_GIT_URL_* variables.
Added FAQ on non-use of import and using in functions
Merge pull request #7553 from simonp0420/using_func

Added FAQ on non-use of import and using in functions
rational: print ±1//0 as ±1//0 instead of ±Inf
Merge pull request #7513 from mbauman/filt

RFC: filt improvements
Fix typo and enumeration list in README.windows.md
Merge pull request #7557 from scidom/patch-1

Fix typo and enumeration list in README.windows.md
distinguish between libraries and bundled exe's
Support for icc and ifort.
It is possible to use MKL's FFT through their FFTW wrappers
Clean up compiler warnings
Use rem_pio2 from openspecfun. Allows other libms to be used again.
Fix #5365