Make add!! decide if mutable or not

[oxinabox]

Closes #232

[oxinabox]

Perhaps this is cheap enough to do it outside of debug_mode?
Maybe if we find outselves having to tell people to turn on debug_mode to find errors we should look into that?
Or maybe we should look into it now.
What do you think?

[oxinabox]

Does this invalidate
https://github.com/JuliaDiff/ChainRulesTestUtils.jl/blob/master/test/check_result.jl#L10
as a test?
I don't think it does

It does invalidate this one
https://github.com/JuliaDiff/ChainRulesTestUtils.jl/blob/d369e3fff2b8c6780024b5df2150e6556bd3ec67/test/testers.jl#L79

[nickrobinson251]

A couple suggestions for improvements on the user-facing side, otherwise looks really good

[nickrobinson251]

detailing what overloads need to be written to fix this

Is this true? this suggest to me it'll print something like

"""
If `Foo` can be updated in place, define `ChainRulesCore.is_inplaceable_destination(::Foo) = true
"""

But as far as i can see that's not in the error message, or did i miss it (or misread this comment)?

[oxinabox]

This is actually documenting something older.
Its unrelated to the main content of this PR.
Not inplace add but simply +
Its this message

ChainRulesCore.jl/src/differentials/composite.jl

Lines 269 to 284 in a3e76b1

	function Base.showerror(io::IO, err::PrimalAdditionFailedException{P}) where {P}
	println(io, "Could not construct $P after addition.")
	println(io, "This probably means no default constructor is defined.")
	println(io, "Either define a default constructor")
	printstyled(io, "$P(", join(propertynames(err.differential), ", "), ")", color=:blue)
	println(io, "\nor overload")
	printstyled(io,
	"ChainRulesCore.construct(::Type{$P}, ::$(typeof(err.differential)))";
	color=:blue
	)
	println(io, "\nor overload")
	printstyled(io, "Base.:+(::$P, ::$(typeof(err.differential)))"; color=:blue)
	println(io, "\nOriginal Exception:")
	printstyled(io, err.original; color=:yellow)
	println(io)
	end

I just realized when writing this this PR that I there was no list of what Debug Mode did,
so I couldn't add the Inplace behavour to it.

[oxinabox]

Those integration test failures look like they need to be looked into