trees: implement more TreeSet helper methods

[shoenig]

This PR implements the following helper methods on TreeSet

• Below - set of elements strictly below a value
• BelowEqual - set of elements below or equal to a value
• FirstBelow - first element strictly below a value (if exists)
• FirstBelowEqual - first element below or equal to a value (if exists)
• Above - set of elements strictly above a value
• AboveEqual - set of elements above or equal to a value
• FirstAbove - first element strictly above a value
• FirstAboveEqual - first element strictly

Closes https://github.com/hashicorp/go-set/issues/37 Closes https://github.com/hashicorp/go-set/pull/38

[vanodevium]

@shoenig Please make release. I wanna update my mod file in classic way, by @latest

[shoenig]

Hi @vanodevium, I just created a release for v0.1.12

BTW in case you are still interested in contributing, I also filed #45, #46, #47 for creating examples compatible with Go doc, which might be a nice place to get started