New externals based vector and matrix libraries

[br4sco]

This PR adds new external-based libraries for matrices and vectors. It also refactors and extends the external array library.

arr-ext.mc
This library implements (Ocaml native) and (Bigarray) 1D arrays. The Bigarray-based, or ExtArr arrays as they are called, can hold either double or single precision floats.

cblas-ext.mc
This library implements a (incomplete) high-level CBLAS interface based on ExtArr arrays. It will pick the right implementation based on the kind of the ExtArr array (single or double precision right now).

The CBLAS routines are flexible and take many arguments. Therefore I added a record with default arguments for convenience. They are named the same as the function but suffixed withArg. For example, you call the CBLAS routine gemm that performs the computation y := alpha*A*x + beta*y as:

cblasGemv { cblasGemvArg with beta = 2. } m n a x y

where we change the default value of beta = 0.0 to 2.0.

vec-ext.mc
Implements ExtArr arrays as vectors. Currently, it only implements the operations necessary to define a vector space (addition and scalar multiplication). The implementation uses cblas-ext.mc.

mat-ext.mc
Implements matrices based on ExtArr arrays. It implements operations over matrices using a combination of
cblas-ext.mc and external operations implemented in Owl. All but one operation (matrix exponential) can be used in place or in a referential transparent manner, where the latter performs a copy/data allocation.

Updated Microbenchmarks

I updated the Microbenchmarks and added benchmarks for mat-ext.mc. Some relevant results are as follows on my machine (matrix multiplication with matrices of size 100x100):

for t in mul mul_naive; do echo $t; ./run -e 1245 matrix_$t; echo ""; done 
mul
1. Miking compiler:         0.014660s  0.016695s
2. OCaml native:            0.019750s  0.020799s

mul_naive
1. Miking compiler:         4.854387s  4.443769s
2. OCaml native:            4.039013s  3.961367s

The benchmark mul compares matMul from mat-ext.mc with Owl.Mat.( *@ ) (matrix multiplication). The benchmark mul_naive compares naive matrix multiplication in a triple for-loop between matrices from mat-ext.mc and Bigarray.Array2 arrays.

For comparison, these are the times for naive matrix multiplication using Bigarray-based tensors:

1. Miking compiler:         9.424949s  9.459076s

and native OCaml arrays

2. OCaml native:            0.379540s  0.375307s