Numerical Analysis Module

Since Swift Numerics will be heavily based on numerical computing and math, I want to make this proposal for an add-on module on [**numerical analysis**](https://en.wikipedia.org/wiki/Numerical_analysis). Example use cases include (but are not limited to) the following:

* Finding the roots (or zeroes) of continuous functions
* Interpolating polynomials
* Approximating the definite integrals of continuous functions
* Solving linear systems
* Solving initial value problems

NOTE: `Complex` numbers are required for implementing [Muller’s method](http://mathworld.wolfram.com/MullersMethod.html).

P.S.: As someone with a background in applied mathematics, thank you @stephentyrone for introducing this wonderful Swift package!

This is a broad proposal for a numerical analysis add-on covering roots, interpolation, integration, linear systems, and initial value problems. No files, tests, or entry points are named, so first establish the module scope and its relationship to Complex. Done requires an agreed design and an implemented, tested set of numerical-analysis capabilities.

Written by the indexing model from the issue text.