Add ability to set assertions for values on the stack and assumptions for data placeholders

[dgpv]

No description provided.

[dgpv]

Should fix #15

[dgpv]

This is incomplete because no automatic tests are added yet. For now it just passes a few manual tests I've done, and seems to work.

It might take a while until I find time to work on automatic tests, and meanwhile this might be interesting to those who wish to experiment with these assertions, so I'm putting this PR to public.

@ajtowns you might want to check this out - IIRC you mentioned that something like this could be useful to you.

I would appreciate any comments regarding the usefulness of this limited form of assertions and the ergonomics of their usage.

Later, #14 can be resolved by using assertions to place constraints on args of PICK or ROLL and when these args are analyzed, just generate their possible values repeatedly until certain specified limit. Afterwards, use the generated values to create separate execution paths.

[dgpv]

The format for the assertions is described in the updated README.md

[dgpv]

Added distinction between assumptions and assertions

// bsst-assume($name): ... and // bsst-assume-size($name): ... set assumed constraints on data placeholder $name.

The README section about assertions and assumptions is updated accordingly.

No tests still.

[dgpv]

Rebased on main, squashed.

Started adding tests.

[dgpv]

It seems to be relatively easy to add an ability to use data placeholders and data references within assertions (like, // bsst-assert: >=&someref <$somedata). But I hesitate to add this, as I fear feature creep. Data placeholders or references with only comparison and without full-blown arithmetic on them seems not very useful, but I did not want to add expression parser, I want to avoid code bloat and any external dependencies. I expect that complex assertions can be dealt with via analysis plugins (#19) if these are implemented.

Still, if someone can show how assertions that use data placeholders/references without any arithmetic on them might be practically useful enough to have them available in the "simplified assertions", I might consider adding this feature

[dgpv]

By the way, using data references with asserts might be useful in another way, to refer to previous values on the stack, like this:

         // bsst-assume($a): le64(20) le64(21)
        $a DUP
        le64(1) ADD64 VERIFY le64(3) DIV64 VERIFY
        SWAP // =>remainder
        le64(0) EQUAL VERIFY
        // bsst-assert(&remainder): le64(0)
        // bsst-assert-size(&remainder): 8
        // bsst-assert: le64(7)
        DROP

Since it has obvious practical value, I have added this feature (// bsst-assert(&ref): / // bsst-assert-size(&ref):)

[dgpv]

Added tests.

[dgpv]

Added ability to apply assertions to witnesses: // bsst-assert(wit0): >3, // bsst-assert-size(wit1): 64 65 etc.

Can be used to ensure that restrictions that the script put on its witnesses are as expected