Remove some automatic conversion and comparison

[fingolfin]

• Remove methods that allow comparing an object of type CxxRef{T} with one of type T. Note that other Ref types in Julia don't provide such methods either.
• Remove convert methods from SmartPointer instances to to other types.

In both cases, the goal is to avoid invalidations. To give an idea of how bad the removed methods were, consider the number of invalidations as computed with this snippet in a fresh Julia session:

using SnoopCompileCore
invalidations = @snoopr using CxxWrap;
length(invalidations)

Before this patch, we get 20819 invalidations with Julia 1.8.4, and 13863 with Julia master.

After this patch, this is down to just 2695 in 1.8.4, respectively 1480 in Julia master.

The main issue with this patch is that it may break code relying on these convenience methods. However, the resulting failures should be easy to fix, and the fixed code should work fine with older CxxWrap versions. I would nevertheless suggest to mark this as a breaking change by following the appropriate semver rules.

This makes further progress towards resolving issue #278.

Resolves #313
Closes #334

[fingolfin]

@barche I had to make minor modifications to the test suite, but now it passes. I also verified that the following packages using CxxWrap directly or indirectly still pass their test suites:

• Casacore.jl
• Oscar.jl
• Polymake.jl
• Singular.jl

I had trouble running the test suites of openPMD.jl and OpenSpiel.jl, but already without this PR, i.e. with the CxxWrap main branch (and also the latest release). This may be due to me running the tests on a mac, though... Weird.

In any case, based on my findings so far, I think it should be fine to merge this as long as the semver of the package is increased afterwards to indicate to a breaking change.

[barche]

Thanks for this, causes no issues at all for QML.jl either.

[fingolfin]

Great! Now just need CxxWrap.jl 0.13 to be released... well, and I am off to tracking down the next source of invalidations ;-)