script: remove total size constraint

It's not necessary to set an upper/lower bound on addition/subtraction
of operands. CScriptNum should only throw when trying to use an overflown
vector input.

Convert the += and -= guards to asserts against int64 overflow, since that
should be impossible during script evaluation.
script: test: remove an unnecessary guard

Many assumptions in these tests are now wrong, since the max size constraint has
been lifted. Removing this one allows the tests to pass.

TODO: Remove the other now-invalid assumptions in the tests.