Improve performance of implicit search

[jvican]

These are the times we are looking for an implicit of type T in the implicit search for our test suite (which, btw, is huge):

Type	Times
Any	1420
String	1456
Int	1677
Ref	2338

These stats lay the ground for the upcoming improvement in the implicit search that hopefully will address the performance issues that we face right now, especially large compilation times. I think that there's room for improvement and I'm working tirelessly on it.

[jvican]

Current:

Type	Times
Any	168
String	0
Int	0
Ref	0

I still have to figure out why Any is still being invoked through the macro generation. This hints that the compiler cannot find it in scope. This is probably caused by some test cases not importing AnyPicklerUnpickler from the whole Defaults (we have some tests that cherry-pick the imports) and therefore pickling has to create it from scratch.

My last changes (in my private repo) decrease the compilation time by a half (in my computer the speedup is ~3x). There are still some more improvements coming that will improve preferringAlternativeImplicits.

[jvican]

It also improves #342. I don't own a MBP, but in my Thinkpad x201 which is about 5 years old, the example in the issue consumes ~2s to compile. I think I could cut it even more with the upcoming changes.

[jvican]

I'm going to put off the next improvements in the implicit search for the next PR, since it's not high priority.