Quasi-comprehensive BigDecimal soundness/correctness fix.

This fixes issues SI-6153, SI-6173, SI-6456, SI-6699, and SI-8116, along with a number of other similar possible issues.

Relevant changes include
  * Changes to avoid heap explosion when working with BigInt
    - to isWhole
    - to hashCode
    - to equals
    - to BigInt's equals
  * Changes to enable equality matching hashCode
    - Only for sufficiently small BigInt
    - For identical values with different precision
  * Changes to isValidDouble
    - Takes precision into account now
    - New methods added to test whether even if the Double is not represented exactly, it's a representation of a certain type
    - New companion methods added to allow intended expansion of Double (binary/decimal difference)
  * Changes to constructor
    - Null arguments are not allowed (these can throw NPEs later at awkward/unexpected times)
  * New JUnit test to test all these things
  * Fixed existing tests to expect new behavior
  * Modified scaladocs to explain the issues
  * Deprecated problematic methods
  * Made application of MathContext more consistent (it is where you expect it and not where you don't)

These changes are coordinated, for the most part, hence the monolithic
commit.