Defines +,- for Triangular matrices

Fix #5927
 Fix issues in determining return type of sum/prod along dim

- Fix reductions along dimensions of Union typed arrays
- sumabs/sumabs2 for Complex64/Complex128 should return Float32/Float64
Merge pull request #7539 from JuliaLang/sjk/reducedim_type

Fix issues in determining return type of sum/prod along dim
windows: download links
WINDOWS: fix some other issue mentioned in #7082 where NamedPipes can freeze up
link "fetch" too
Merge pull request #7624 from waldir/patch-1

link "fetch" too
New version of mingw64-libexpat
Merge pull request #7627 from ronisbr/master

New version of mingw64-libexpat
Quick fix to metaprogramming.rst
Merge pull request #7629 from kersulis/patch-1

Quick fixes to metaprogramming.rst
parse ¬ (\neg) as a prefix operator
Merge pull request #7630 from stevengj/neg

parse ¬ (\neg) as a prefix operator
Add a brief explanation of n:n-1 as an index

Update splice! docs to describe insertion without removal of any elements.
Merge pull request #7631 from JuliaLang/kms/doc_empty_range_as_index

Add a brief explanation of n:n-1 as an index
Don't be so picky about patchelf version, as there are now newer versions floating around
Fix type inference for \
add some precompile items to help Pkg.update speed
In Makefiles, use $(shell which ...) not $(which ...)
call update_all_fptrs earlier, to avoid some redundant compilation