Remove Let, add a 'mem' operand class instead

[Hugobros3]

This removes the Let node and associated machinery (Variable, Block, ...), instead transitioning shady to be a unscheduled IR, à la Thorin. A "scheduler" analysis was added so printing and backends can place instructions.

To support this change, a number of additional refactors have been applied:

• Anonymous lambdas/Cases have been removed and basic blocks are used instead.
• The structured control flow instructions have been replaced with tail/terminator variants, which makes the CFG analysis simpler and probably sound. The new structured constructs have a "tail" block instead of relying on the outer Let's.
• Primops are treated as side-effect free, which necessitated the removal/migration of all the ones that used to have side effects into either new nodes, or ...
• Extended instructions: They work similar to primop but use a string+id, which maps 1:1 with SPIR-V's extended instructions. This is used to embed SPIR-V directly inside shady.
• Frontend nodes are gone: Nodes such as Unbound or Dereference have been removed and a private set of extended instructions

Pending/TODO:

• Mem2Reg pass has been deleted, must be rewritten
• opt_stack: ditto
• simt2d is deleted with no immediate plans to bring it back
• switch to if-tree lowering is broken
• l2s is broken
• s2s is broken