Refactoring sparse_hash_map::emplace's helper functions

[brandonfoong]

Thanks for merging #48.

In order to minimize the changes made, I implemented the helper functions (emplace_at, emplace_noresize, set_inplace) separately from the existing helper functions for insert (insert_at, insert_noresize, set).

However, dense_hash_map only has one set of helper functions (insert_...), which are used for both insert and emplace. Would it be ok for me to do something similar for sparse_hash_map (i.e. only keep 1 set of helper functions to avoid repetition)?

[PhilipDeegan]

if you think it's better go for it