refactor: use patterns in more of the ast

[tjjfvi]

• lambdas and lets can now bind simple patterns (e.g. @(a, b) bod or let ((a, b), (c, d)) = val; nxt)
  • Term::Dup and Term::Ltp are now part of Term::Let
• these patterns can now bind scopeless variables (so e.g. let $x = ... is valid)
  • Term::Chn is now part of Term::Lam
• tuples can now be tagged, and use the same AST node as superpositions
  • tup/sup/dup are all collectively called fan
  • the logic for these is nearly identical for the vast majority of the code
• erased lets are no longer elided, fixing some miscompilation bugs

(even though this added features, the line count is net negative!)

[tjjfvi]

(this is a modern version of the original pattern-refactor started by @FranchuFranchu (cf1bc25))

[developedby]

Does hvm-core completely ignore the value of labels? As in, it doesn't ever look at the meaning of any specific values, just compares them between nodes.

[tjjfvi]

correct