Implementation of deleteat!
Refactor splice!, delete!
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
normalize all flisp symbols to NFC (fix #5434)
No more ImaginaryUnit: `const im = Complex(false,true)`.

This change requires giving multiplition by Bools special behavior.
Approximately, `true * x = x` and `false * x = zero(x)`, but a bit
complicated for the sake of promotion, the only non-trivial example
of which is `Bool * MathConst`, which is promoted to Float64.

Idea originally due to @GunnarFarneback:

    https://groups.google.com/forum/#!topic/julia-dev/VkGrqnrAdaY
    https://github.com/JuliaLang/julia/issues/2980
    https://github.com/JuliaLang/julia/issues/3728
base/complex.jl: show Complex{Bool}s as e.g. `Complex(true,false)`.
base/multimedia.jl: show method for MIME types.
base/complex.jl: additional incidental cleanup and simplifications.
Improve special matrix +, - operations

Specifically, adds missing + and - methods between
(Bidiagonal, SymTridiagonal) and
(Diagonal, SymTridiagonal)
Remove deprecated syntax in test/unicode.jl
NEWS update
Merge branch 'master' of https://github.com/JuliaLang/julia
Defines *(Number, Tridiagonal) method

Also tidies up SymTridiagonal methods