intrinsics module with alternative implementations

[jalvesz]

Add intrinsics module containing replacements for intrinsic function where some feature is found interesting: faster implementation, better accuracy, both simultaneously.

This PR follows the discussion in discourse https://fortran-lang.discourse.group/t/lfortran-now-supports-all-intrinsic-functions/8844/41 and it's based on https://github.com/jalvesz/fast_math

• sum: 2 options (stdlib_sum and stdlib_sum_kahan)
• dot_product: 2 options (stdlib_dot_product and stdlib_dot_product_kahan)

cc: @fortran-lang/stdlib @perazz @certik @jvdp1

[jalvesz]

One philosophical question: should the fsum interface be renamed to sum to enable direct replacement of the intrinsic? Keep this name? Or yet something like stdlib_sum? (Same for fprod->dot_product)

Regarding the kahan versions, given that the accuracy gains are close between the pure chunked version and the kahan one, I'm wondering which level of support should be enabled to switch between them?

[perazz]

IMHO shorter names are better, and don't see a problem if they overlap with the intrinsics. First, because one can always pick the right version:

use stdlib_intrinsics, only: dot_product

vs.

! Force using intrinsic
intrinsic :: dot_product

And then because they can be augmented by more/different arguments

c = dot_product(a,b) ! intrinsic
c = dot_product(a,b,mode='kahan') ! stdlib
c = dot_product(a,b,mode='blocked') ! stdlib
...

I find this more elegant and definitely not confusing.
This PR also reminds me that it would be worthwhile to also augment the matmul intrinsic via calls to the gemm backend