Add support for constant patterns

[yannham]

This PR adds support for constant patterns, that is patterns matching specific booleans, strings, numbers or the null value. This allows to write for exampe:

match {
  "foo" => ...,
  "foo/bar" => ...,
  other => ...
}

Or

match {
  {enabled = true, ..rest} => code_path_when_enabled rest,
  {enabled = false, ..} => 'Error "Is not enabled!",
}

For destructuring, the semantics is the same as for enum tags - which are actually constants as well, but were just supported as a side effect of supporting eneral enum patterns - : a constant pattern forces the value and works as an assertion.

[jneem]

Is the type comparison an optimization? It looks to me like BinaryOp::Eq already handles mismatched types.

[yannham]

No, it's because we can't compare functions. So we want to avoid this runtime error and eschew the comparison instead:

error: cannot compare values for equality
  ┌─ <repl-input-1>:1:1
  │
1 │ (fun _x => null) == (fun _x => null)
  │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  │ │
  │ an argument has type Function, which cannot be compared for equality
  │ problematic argument evaluated to this

Since we have to do a pre-condition check anyway, instead of checking if one is a function, it's just simpler and probably faster to just avoid comparison for any value that doesn't have the expected type to begin with.