Support overloading Objective-C methods based on static/instance.

[rolfbjarne]

Two Objective-C methods can be named identically as long as one is static and
the other instance.

We must support this since Apple did just this (in the NSItemProviderReading /
NSItemProviderWriting protocols).

We solve it by prepending a '+' or '-' to the selector when hashing it (to
determine selector uniqueness, and to look the method up again at runtime).

[monojenkins]

Build failure

[spouliot]

minor: comment looks incomplete

[rolfbjarne]

Fixed.

[spouliot]

That seems to have the potential to create a lot of strings. Could we (later) try to:

• lookup different static/instance collections ? or
• only prepend "+" for static (and leave the instance without a sign) ?

[rolfbjarne]

I chose the implementation that I thought would have the lowest minimum memory requirements.

• Having two different dictionaries, one for static and one for instance, would use more memory since there would be two dictionaries instead of one.
• The strings are transient, the GC will collect them, and as such they don't contribute to the minimum memory requirements.
• I could use a different key type (Dictionary<Tuple<string, bool>, ObjCMember for instance), but this means AOT-compiling a Dictionary with a struct as a key, which would increase code size (unless we already AOT-compile such a dictionary for something else).

I considered prepending + for static methods only, but then I thought of this (admittedly pathological) case:

[Export ("+foo")]
public void Foo () {}

which won't work with the static registrar, but works with the dynamic registrar (which is a use-case for Xamarin.Mac).

Finally I came to the conclusion I might be able to remove the whole problem when I make the static registrar do more at build time (see bug #56022).

[monojenkins]

Build success