QDrift C API Implementation

[Didgety]

Summary

This pull request implements QDrift Trotterization within the C circuit library.

Details and comments

As part of the implementation, some things were made public:

pauli_evolution module from the Rust circuit_library crate, a type alias within that module Instruction, and a function sparse_term_evolution

The implementation was loosely based on the Python version, but is grounded in the mathematics from the paper: https://arxiv.org/abs/1811.08017

Tests were created for valid observables, invalid observables, gate count scaling, invalid repetitions, observables containing projectors, and global phase.

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[Shobhit21287]

Thank you for your contribution! Although it seems that the .unwrap() here shouldn't error out, it's probably still good practice to use except("err message") so that the error (if it occurs) is well-defined.

[Didgety]

Replaced that with a match statement that will return an exitcode on failure, rather than panicking. Also updated approx_real to remove the panic and return an exitcode instead in 5f588d8