VavrAssumptions public methods return package-private types

[kennymacleod]

The various assumeThat methods in the VavrAssumptions class return package-private types rather than the public types returned by VavrAssertions.

For example, whereas VavrAssertions#assertThat(Map) returns a public MapAssert type, VavrAssumptions#assumeThat(Map) returns a package-private AbstractMapAssert type.

As a result, none of the assumeThat methods are usable from code outside of that package, since they won't even compile.

[mchmielarz]

Good point @kennymacleod! Will fix it!