ForeignFunctions package for Macaulay2

We introduce the ForeignFunctions package for Macaulay2, which uses libffi to provide the ability to call functions from external libraries without needing to link against them at compile time. As examples, we use the library FFTW to multiply polynomials using fast Fourier transforms, call a LAPACK function to solve a general Gauss-Markov linear model problem, and use JIT compilation to compute Fibonacci numbers.