Make pivot_longer and pivot_wider generic

[hadley]

[chester-gan]

Specific feature request: please enable pivot_longer's values_ptypes argument to handle exceptional cases whereby the input columns used to generate the variables generated from values_to are not of the same type.

[hadley]

@chester-gan please file an issue with reprex; commenting on unrelated issues is not a good way to get help.

[chester-gan]

@chester-gan please file an issue with reprex; commenting on unrelated issues is not a good way to get help.

Noted, issue created under reprex.