Extra Utilities for Linear Haskell, missing in linear-base

Missing utilities for Linear Haskell, for array, witness token, memory, parallelism, and streaming.

• linear-witness: Linear Witness Tokens. Tentative workaround until we have Linear Constraints in GHC.
• linear-array-extra: Missing arrays, such as borrowable arrays, unboxed/storable/primitive arrays/vectors and more.
• linear-memory: Off-heap allocation utilities.
• linear-parallel: Parallelism combinators for pure linear expressions with side-effects.
• linear-fft: Demonstration implementation of pure, in-place and parallel Fast Fourier Transformation, using linear-parallel and linear-array-extra.