sparse_hash_map: Replace insert function series with emplace equivalents

[brandonfoong]

Closes #49.
This PR replaces the insert and set series of functions in sparse_hashtableand sparsetable with their emplace and set_inplace equivalents. The helper functions are now named insert.../set, but their behaviour follows that of emplace.../set_inplace.
This is done to make it more consistent with the implementation of dense_hashtable.