lib: overhaul Requalify, add arch variants

* add warnings when exporting a name that already exists in theory
  context, suppressable by `(aliasing)` option
* add `arch` variants of requalify commands that implicitly add the
  value of L4V_ARCH before whatever you give them, with optional
  suffixes for abstract (A) and Haskell (H) spec global naming.
* write hopefully-understandable documentation with commented examples

Signed-off-by: Rafal Kolanski <rafal.kolanski@proofcraft.systems>