[D-runtime] Merge core.sync package into a single module?

[Alex Rønne Petersen]

Hi folks,

I've been thinking that it might be a good idea to merge the entire
core.sync package into one module for a couple of reasons:

1) The modules are small.
1.a) I don't think they really need an entire package and several
files to be navigable (at least to me, it makes it more annoying to go
through them).
1.b) The idea seems to be class-per-module which is quite unusual in
typical D code.
2) It'll be easier for the user to just import one module, instead of
several depending on what synchronization primitives they need.
3) It's consistent with the general interface in core.* (the only
nested modules/packages we have are in core.sys and core.stdc).

Thoughts?

Regards,
Alex