Some of the functions in difficulty.go are not dependent on any external
functions and they are needed to introduce testing code for the
invalidateblock and reconsiderblock methods that are to be added on in
later commits. Having the workmath package let's us reuse the code and
avoid dependency cycles.

The existing functions that were exported already (HashToBig,
CompactToBig, BigToCompact, CalcWork) are still kept in difficulty.go
to avoid breaking external code that depends on those exported
functions.

spendableOut and the functions related to it are is moved to package
testhelper and are exported.  This is done to make the code reusable
without introducing an import cycle when the testing code for
invalidateblock and reconsiderblock are added in follow up commits.

solveBlock is moved to testhelper and is exported.  This is done so that
the code can be reused without introducing import cycles.  The testing
code to be added in alter commits for invalidateblock and reconsider
block will use SolveBlock.

The variables are moved to testhelper so that they can be reused in the
blockchain package without introducing an import cycle.  The testing
code for invalidateblock and reconsiderblock that will be added in later
commits will be using these variables.

uniqueOpReturnScript is moved to testhelper and is exported so that the
code and be reused in package blockchain without introducing import
cycles.  The test code for invalidateblock and reconsiderblock that are
gonna be added in later commits uses the functions.

standardCoinbaseScript is moved to testhelper and is exported.  This
allows test code in package blockchain to reuse the code without
introducing an import cycle.  This code is used in the testing code
for invalidateblock and reconsiderblock that's added in the later
commits.

createCoinbaseTx's code is refactored out and placed in testhelper
package and is exported so that callers in package blockchain can reuse
the code without introducing import cycles.  The test code for
invalidateblock and reconsiderblock that'll be added in later commits
make use of this code.

createSpendTx is moved to testhelper so that the function can be used
for callers in package blockchain without introducing import cycles.
The test code for invalidateblock and reconsiderblock that are going to
be added in later commits make use of this code.

The block generating functions here allow for a test to create mock
blocks.  This is useful for testing invalidateblock and reconsiderblock
methods on blockchain that will be added in later commits.

InvalidateBlock() invalidates a given block and marks all its
descendents as invalid as well. The active chain tip changes if the
invalidated block is part of the best chain.