[pallas] Sketched an `mpmd_map` primitive
The new primitive is similar to `core_map` in that it it allows to map a
function over a mesh. However, unlike `core_map`
* it accepts multiple mesh-function pairs;
* it does not currently support closed-over values, including refs, and
* it uses value semantics, so the primitive accepts and returns values.
Despite the name, though, there is nothing MPMD about it just yet. The
lowering only supports a single mesh-function. This limitation will be lifeted
if follow up changes.
The primitive is used in the `default_mesh_discharge_rule`.
PiperOrigin-RevId: 871266518
