Add support for trace inputs to PIL

[qwang98]

Add support for trace inputs to PIL. Currently, PIL supports user input via the command line or Rust APIs for the command line. However, it only supports input for one variable (column). Because Chiquito circuit might provide inputs for multiple variables via trace (e.g. the MiMC7 circuit), we can't use use PIL's Rust API as a generic solution for providing inputs. The only solution is to input statements in the format of ISFIRST * ([[input_variable]] - [[input_value]]) = 0. An issue with this approach, however, is that trace is too general and doesn't associate input values to signals, so we might need to create an alternative trace function or mode that does this. All of the above is due to the fact that PIL only supports automatic witness inference and cannot feed in external witness.

Depends on #153 merged