Remove `tidyr` dependency by re-implementing `filldown` function

[yanlesin]

fill function is the only dependency in the package that requires tidyr. It is possible to include cpp filldown from tidyr as non-exported function, in order to remove dependency.
The remaining dependencies will be pdftools and rvest.
rvest changed in the past and broke functionality and should be considered for removing as dependency.
pdftools - ok to have it as dependency.

[yanlesin]

filldown in tidyr is replaced now with functions from vctrs. Will investigate if I can re-implement vctrs version

[yanlesin]

Will close this issue for now - recent profiling of function show almost no impact from use of fill function.