Buck build

[njlr]

This PR adds support for builds using Buck. I wrote this build file because I am currently porting mongo-cxx-driver to Buck, and this is a dependency.

Even though core is header-only, the advantage of having a Buck file is that it describes which header files make up the library in a format that is human and machine readable. This description makes it is easier to integrate into another project because Buck will wire-together the include paths in an intelligent way.

To "build" with Buck:

buck build :core

To run the tests:

buck run test/:algorithm
buck run test/:any
buck run test/:array
# etc... 

The existing CMake build is unchanged; the two can coexist peacefully 😊

[CLAassistant]

All committers have signed the CLA.

[bruxisma]

Hi, I wanted to let you know I've seen this PR but I'm currently recovering from surgery for the next few weeks so it'll be a while before it gets integrated.