Introduce 64 bit arithmetic expressions

[sanket1729]

• Adds unit and integration tests
• Does not have integration tests for issuance/re-issuance because creating those is painful on the rust side. Please carefully review the Script translation of those fragments

[apoelstra]

Overall this looks really great! The last commit makes me worry that the code will start to become unweildy the more things we add to instrospect, but it's fine to clean that up in a later PR.

One thing I don't fully grok is why we always have the user provide a value then VERIFY that it matches the introspected value. Couldn't we just let the user do this herself using the Arith::Eq fragement, if needed?

[sanket1729]

@apoelstra

user provide a value then VERIFY that it matches the introspected value.

I don't fully understand this. The user does not provide a value. All values are taken from transaction introspection or are fixed constants. The VERIFY in the inner fragments asserts that the value is explicit. We need this to compose with other arithmetic operations.

Recall that INSPECTXXXVALUE pushes two things on stack . We are asserting the prefix in the inner fragment. And we are checking the value equality in the Eq fragment along with overflow check.

[sanket1729]

The last commit makes me worry that the code will start to become unweildy the more things we add to instrospect, but it's fine to clean that up in a later PR.

FTR, I don't see us adding any more things. But I also don't know any way to clean up these things.

[apoelstra]

The VERIFY in the inner fragments asserts that the value is explicit. We need this to compose with other arithmetic operations.

Oh, derp, thank you. I forgot this.

FTR, I don't see us adding any more things. But I also don't know any way to clean up these things.

Well, it adds a whole bunch of function parameters that maybe could be simplified a bit with a TxEnv-like structure.