Implement `checkbounds` with no args + with kwargs

Closes #1156. Prior to this, any call to `checkbounds` with either no arguments or with keyword arguments would error. On a pragmatic note, I'm not sure whether you would like PR submitters to bump the patch number + add a changelog note. Over on the Turing.jl ecosystem, we do that with every PR, so I'm very happy to do it here; let me know!