Rewrite core internals

[bdsoha]

Rewrite core internals in a more object oriented fashion.

• Move the responsibility for evaluating (comparing actual and expected) from BaseExpectation to a new base class

class BaseMatcher:
    def message(self, *, actual=None, **kwargs) -> str:
        ...

    def _match(self, *, actual=None, **kwargs):
        ...

    def _negate(self, *, actual=None, **kwargs):
        ...

    def __call__(self, *, actual=None, **kwargs) -> bool:
       ...

• Rewrite equal as matcher #37
• Rewrite be as matcher #38
• Rewrite contain as matcher #39
• Rewrite be_contained_in as matcher #41
• Rewrite be_empty as matcher #40
• Rewrite be_true as matcher #42
• Rewrite be_truthy as matcher #43
• Rewrite be_false as matcher #44
• Rewrite be_falsey as matcher #45
• Rewrite be_of_type as matcher #46
• Rewrite inherit as matcher #52
• Rewrite greater_than as matcher #51
• Rewrite greater_than_or_equal as matcher #50
• Rewrite lesser_than_or_equal as matcher #49
• Rewrite lessert_than as matcher #48
• Rewrite be_numeric as matcher #47
• Rewrite filesystem as matchers #53
• Rewrite functions as matchers #54