Second edition solutions

• Not all changes are shown in this Release Notes; this is a summary.
• Minor prose changes for clarification throughout.
• Corrections to prose references to line numbers in examples.
• Minor modifications to exercises to conform to solutions.
• Changed "Royalty" examples/exercises in Imports & Packages;
  "Royalty" used class methods which hadn't been introduced. Changed
  to PythagoreanTheorem.scala and associated files and exercises.
• Added Exercise 10 in "Error Handling with Exceptions" to give a
  basic introduction to the "finally" clause.
• Rewrote "Installation" atoms to reflect the way the examples come
  packaged from Github, and for better clarity and accuracy.
• Rewrote exercises 5 and 6 in "Polymorphism"
• Rewrote all exercises and solutions for "Composition"
• Rewrote exercise 3 in "Type Parameter Constraints"
• Rewrote exercise 4 in "Building Systems with Traits"
• Added exercise 4 in "Combining Sequences with Zip"
• Rewrote exercises in "Maps" to focus on Maps (rather than Sets).
  Added Exercise 7 to demonstrate case classes as keys.
• Rewrote most of the exercises in "References and Mutability"
• Removed superfluous exercise in "Pattern Matching with Tuples"
• Removed old exercises 3 and 4 from "Custom Error Reporting" because
  they were inappropriate. Added new exercise 3.
• Improved exercise 3 in "Design by Contract"
• Removed exercise 1 in "Logging" (it didn't seem that helpful)
• Changed a couple of exercises in "Extensible Systems with Type Classes"
  to make the solutions better.