script: add CScriptNum class

This class holds an int64_t and replaces the use of CBigInt for script
integrals.
script: switch to CScriptNum usage for scripts
script: switch outside users to CScriptNum
script: remove bignum dependency
script: add additional script tests
script: Add test for CScriptNum

Because this class replaces some usages of CBigNum, tests have been added to
verify that they function the same way. The only difference in their usage is
the handling of out-of-range numbers.

While operands are constrained to [-0x7FFFFFFF,0x7FFFFFFF], the results may
overflow. The overflowing result is technically unbounded, but in practice
it can be no bigger than the result of an operation on two operands. This
implementation limits them to the size of an int64.

CBigNum was unaware of this constraint, so it allowed for unbounded results,
which were then checked before use. CScriptNum asserts if an arithmetic
operation will overflow an int64_t, since scripts are not able to reach those
numbers anyway. Additionally, CScriptNum will throw an exception when
constructed from a vector containing more than 4 bytes This mimics the previous
CastToBigNum behavior.
Merge pull request #3965

b1fdd54 script: Add test for CScriptNum (Cory Fields)
90320d6 script: add additional script tests (Cory Fields)
05e3ecf script: remove bignum dependency (Cory Fields)
4f497cd script: switch outside users to CScriptNum (Cory Fields)
27bff74 script: switch to CScriptNum usage for scripts (Cory Fields)
48d8eb1 script: add CScriptNum class (Cory Fields)