📦 Change typings-tester for tsd

[Yurickh]

Removing typings-tester in favour of tsd

tsd proved to be useful to our needs in gutenpress, but also we've lost some of the power we had by just defining comments, as we can't combine multiple assertions in the same expression.
e.g.

// before
wrap(wrapper, [
    get('/correct', correctHandler),
    // typings:expect-error
    post('/error', errorHandler),
])

// now
wrap(wrapper, [get('/correct', correctHandler)])
expectError(wrap(wrapper, [post('/error', errorHandler)]))

This is the case since expectError doesn't return whatever it gets, rather it returns void :T
Hence this issue closes #22, but we might want to keep an eye out for better suiting alternatives.

Improving types of fromEntries and mapObejct

As an extra, we're improving the types for fromEntries in @gutenpress/helpers by removing the need to explicitly define the return type, and by being smarter about the returned values. Consequently, we also fix mapObject's types as described in #21, so this issue also closes #21.