it->first/second are always const

[glebm]

In tsl::sparse_map, iterator->first/second result is always const regardless of the underlying constness.
This is different from std::unordered_map, where iterator->first/second result is const only if the map is const.

This is documented in README.md but there doesn't seem to be anything making this impossible in principle.

[firedtoad]

I commit a pull request to fix this problem please review it sooner