add an internal function to replace Main, use it in sysimg.jl

make the Core.Main binding non-constant, so uses of "Main" in general
refer to the *current* Main. Doing "import Main" will give you a
constant reference to the Main at that time.

fixes #5802

related to #1195
Prettified expression printing
make isempty(itr) work for general iterables (as mentioned in #5797)
Merge pull request #5821 from JuliaLang/jb/replace_main

add an internal function to replace Main, use it in sysimg.jl
Merge pull request #5827 from stevengj/isempty

RFC: make isempty(itr) work for general iterables
NEWS for #5827
Merge pull request #5713 from Calder/deparse

Added deparse() command to pretty print expressions
some small string function performance tweaks

document optional length argument to bytestring()
add performance tip on string interpolation. closes #2050
Update NEWS and docs for #5811
Add missing hyperlinks to NEWS
It was getting a little ridiculous having to change the URLs for the win-extras RPMs every couple days.

So now we're mirroring rpms that seem to work, and using jldownload to failover to our cloudfront mirror
julia.h: added dllexports
Merge remote-tracking branch 'upstream/master'
base/primes.jl: generalize primes/mask to BitArray or Array{Bool}.

For smaller numbers of primes, the Sieve of Eratosthenes still seems
to be quite a bit faster. Perhaps we ought to use a polyalgorithm
here, only using the Sieve of Atkin for large N.
Merge pull request #5831 from JuliaLang/sf/mirror_winextras

It was getting a little ridiculous having to change the URLs for win-extras
dllexported jl_field_index
Merge pull request #5830 from cbecker/master

julia.h: added dllexports
help for minmax and extrema. I believe they were mistakenly added to helpdb.jl before.
fix #5836