Feature/improve allocations

[PhenX]

No description provided.

[SebastianStehle]

Looks good in general, but as mentioned a micro benchmark would be helpful. I cannot say, where exactly this would safe allocations. Perhaps less boxing? No idea.

[PhenX]

The allocation that can be prevented with this PR is the one made for the closures here :

return source => converter(actualGetter(source));

The big change is the use of ConvertToProviderExpression instead of ConvertToProvider that makes it possible to compose the getter without any closure.

I'll try to make a micro benchmark

[PhenX]

@SebastianStehle here is the benchmark results, I think it will be hard to to better

[SebastianStehle]

Awesome, good job.