Array extraction: use only byte arrays

[cassiersg]

1. Introduce a new ByteArray theory in eclib (similar to the existing WArray).
   • Have get16_le, get16_be, get32_le..., get16_le_direct, ... (do you agree on the name ?)
2. Introduce a new SubByteArray theory relating two ByteArrays of different sizes.
   • get_sub_direct, set_sub_direct
   • Do we need things like get_sub_scaled16 (which is get_sub_direct, but with index multiplied by 2)?
3. Introduce word extension theories for bijection between words and byte arrays (W8 <-> ByteArray1, W16 <-> ByteArray2...).
4. Extract all jasmin array operations to ByteArray and SubByteArray.

cc @bacelar @mbbarbosa @bgregoir @vbgl