Implementation of deleteat!
Refactor splice!, delete!
Bug fixing only
Replace error() by systemerror() where appropriate

No other semantics should have changed.
More error() -> systemerror()

But this batch needed ios' seek and skip to report whether their failure
has set errno (return -1) or not (-2). I've greped for them in order not
to miss any possible repercussions.
Additional missing systemerror() calls.
Fixes in error-handling of fs.jl.
`flush` return value from Void to Cint
From -one(Coff_t) to -1
Added comments explaining the return -1/-2.
added utf8proc to deps for #5434
make sure llvm finds 2.5 <= python < 3

On some systems python2 doesn't exist (e.g., freebsd10),
or python2 < 2.5 (e.g., centos5).
correct NEWS about readdlm
normalize all flisp symbols to NFC (fix #5434)
promotion: symmetrize promotion rules recursively with promote_type.

If promote_rule(T,S) and promote_rule(S,T) disagree, call promote_type
again on the results to get a single common answer. This can resolve
cases where there is a conflict between two such rules, making the
promotion system a bit more robust.
pretty-pretting of which and methodswith output
math constants: simplify and generalize promotion with constants.
Update contrib/debug_bootstrap.gdb
Merge pull request #4967 from simonp0420/amos_error_checking

Add error checking to Amos package-based Airy and Bessel methods. Correct # arguments in ccall to :zbiry.
further promotion rule simplifications