Support scalar concatenate for strings

[bryevdv]

Currently two string columns can be "horizontally" concatenated (i.e. element-wise). There is not a libcudf concatenate that takes a single scalar prefix and a string column. However it seems that replace_slice could be used to implement such. If this is added, the hypergraph implementation can then be improved to use categoricals by default, to mitigate the risk of running in to global string column limits.