Moving all the core exercises to Catch2.

[arcuru]

Drops Boost for Catch 2.7.0 in the core exercises (after #240 is merged). Depends on #239 getting merged first.

Related to #233.

[elyashiv]

Looks nice. The errors also look quite clear.
I will like to see how hard is it to migrate more complex test case. Two I can think of are handling scoped enums in triangle, and array compression in binary_tree.

[arcuru]

@elyashiv That's a good idea, these are mostly only the simplistic test cases. I'll setup some of the more complex cases in a different PR.

Triangle is pretty easy, there should be only limited difference from my older changes - https://github.com/exercism/cpp/blob/remove_boost/exercises/triangle/triangle_test.cpp

For all the instances of BOOST_REQUIRE_EQUAL_COLLECTIONS, Catch instead relies on using REQUIRE statements and gives pretty good error messages ( It also has Matchers which also has some overlap with that, but those aren't likely to be too useful for the existing tests at least since we're comparing against a fully known output). Most places we use that are just comparing two containers, so we can switch to just using REQUIRE(expected == actual). The trickiest compare is probably going to be the comparison using the iterator in binary_search_tree, which I should be able to do by just building a container out of the iterator. I'll mess around with it to figure out a good solution.

As an example of a failure, I edited one element of a vector and did a comparison to give this output (which is colored too):

exercises/anagram/anagram_test.cpp:42: FAILED:
  REQUIRE( expected == matches )
with expansion:
  { "stream", "maers" }
  ==
  { "stream", "maters" }

===============================================================================
test cases: 10 | 9 passed | 1 failed
assertions: 10 | 9 passed | 1 failed