[VAN-503] Convert to/from i1 as needed

[iangneal]

The logic functions (fr_logic_*), the library functions (like constraints and assertions), and branch instructions require i1 (i.e., boolean) arguments. So, when storing the results of logic operations to a regular variable (which is always a bigint, i.e. i256) or implicitly converting a regular variable into a boolean (e.g., if (foo)), a conversion needs to occur in the LLVM IR. I've implemented the following changes:

• When creating a call, store, or return with boolean arguments, automatically add a conversion to boolean if the provided argument is a non-boolean integer (by adding a computation of val != 0).
• When creating a call, store, or return with an integer argument, automatically add a conversion to the non-boolean integer if the provided argument is a boolean (by adding a zext instruction).
• When creating a branch, ensure that the value is converted to a boolean using the above methods.

I also added 4 test cases (3 from the original issue, one that I added during debugging).

[tim-hoffman]

Thanks! Looks great.