v0.3.1

• Use noexcept operator and simplify documentation (#63)
• docs: Add reference to Vcpkg port (#62)

Introduce from_variant, documentation improvements, and small refactorings

• feat: from_variant: Restrict target type to variant's members
• build: Don't build tests by default and clean up documentation
• feat: Add new operation to extract a variant into a nullable

Use more common names and reduce copies

• feat: Propagate non-member callables by forwarding-references.
• break: Rename combinators to use names more common in the C++ vocabulary.
• break: Drop special syntax for non-static member functions.
• break: Stop using common syntax.
• feat: Compile and test on Windows.

Add more combinators and provide a better API

This pre-release for the unstable API gets us closer to the first stable release that shall happen soon.

Add basic combinators for generic and type-safe composition of nullables

tests: Port test cases to Catch2

And use its BDD-style that fits nicely with Catch2 support for names
that do not need to be valid C++ identifiers.

Given that absent does not use mocks, a simpler framework such as Catch2
might be a better option than GoogleTest right now.

Moreover, the tests are now simplified in order to reduce noise and
improve readability rather than other attributes (e.g. run-time
performance), for instance via passing by-value for lambdas instead of
const ref, etc.