Move everything from java-functional-feature-exercises to java-core-exercises

[tboychuk]

We are going to have one repo for exercises related to Java SE features.

Ask

• Copy all exercisers from java-functional-features-exercises to java-core-exercises
• Rename account-analytics to crazy-streams
• Rename sum-of-squares to declarative-sum-of-squares
• Rename math-functions to lambda-math-functions

Tech. notes

• There should be two PRs for both master and exercise/completed