add infallible serialization functionality

[matty-kuhn]

this work was borne out of the discussion here: #41

the bulk of the changes actually lie in vest/src/infallible.rs, which contains the definitions for secure_serialize and serialize_infallible. all the other changes in the diff lie in the different codegen for different test files

[matty-kuhn]

as mentioned here: #help > vest serialization proof strengthening, i'm planning to narrow the scope of this, to strengthening the postconditions on serialize, instead of adding a new function/trait

[y1ca1]

Thanks for taking the time to do this! The strengthened conditions looks good to me and I agree changing the existing serializer functions would be the easiest path (so no new traits, no duplicate serializing logics, and hopefully relatively small proof changes).

[matty-kuhn]

I added some changes to rework so that instead of creating a new trait, i strengthened the post conditions on Combinator. this breaks some of the code in vest-examples, specifically the ones that have custom Combinator implementations