fakesys: move fakeev to fakesys library

Fakesys is a collection of fake implementations of deep system
things such as libev and libc.

The fake subsystems will provide API just like their original
counterparts (except for function names), but with full control of
their behaviour in user-space for the sake of unit testing.

Fakeev is a bogus version of libev, whose main feature is virtual
time. Fakeev has internal clock, which is fully controllable in
user-space. That allows to roll hours of tests in milliseconds of
real time.

Fakeev is used in SWIM tests, and will be used in Raft tests.

Part of #5303