* add phantom-tagged ExecutionContext and Future to scala-utils concurrent package
* many new operations for Futures
* Future, ExecutionContext combinators from porting ledger-on-sql
- picked from 546b84ab9cdf4de2d93ec5682bdee6cfd6b385f8
* move Future, ExecutionContext companions into normal package
* lots of new docs
* many new Future utilities
* working zipWith
* tests for ExecutionContext resolution, showing what will be picked under different scenarios
* even more tests for ExecutionContext resolution
* tests showing some well-typed and ill-typed Future combinator usage
* no changelog
CHANGELOG_BEGIN
CHANGELOG_END
* missed scalafmt
* one more doc note
* split concurrent package to concurrent library
Co-authored-by: Samir Talwar <samir.talwar@digitalasset.com>
