bug: recursive hints not supported by R1CS solver

[gbotrel]

if we consider h1 and h2, two hint functions.
h1 produces wire w1, h2 --> w2.

Now, if h2 takes as input w1, and w1 didn't appear in a constraint preceding the one solving w2, then the solver needs to solve h1 first when encountering h2, which is not currently the case.

• add simple test case to reproduce the issue
• restore panics in solver when a wire is unsolved (with debug tags only)
• figure out how to deal with recursive hint solving (either when building dependencies in parallel levels, or at a later stage in the solver)