feat(Isla): Support named page-table walks

[febyeji]

Summary

• Support as name annotations and pteN(walk) / tableN(walk) expressions, preserving existing two-argument calls.

• With this, we can refer to translation-table pages by mapping name (which VMSA litmus tests use), for example:

  s1table new_table 0x280000 {
      x |-> pa1 as new_walk;
  }
  x |-> table(table3(new_walk)) at level 2;
  

[tperami]

This adds a lot of complexity for only one test we know of which can easily be written in an already supported way with a 2-line diff. We'll patch the test instead