Fix arrow parsing; avoid extra block nodes in AST

[berekuk]

This fixes #3110, and also fixes #2670, and also simplifies grammar and AST (see tests in diff for examples).

Previously:

$ node dist/cli/index.js parse -e 'x=5'
(Program
  (LetStatement
    :x
    (Block 5)
  )
)

Now:

$ node dist/cli/index.js parse -e 'x=5'
(Program
  (LetStatement :x 5)
)

The arrow bug in particular was caused by parsing let statements as lambdas first, before falling back on arbitrary expressions. I don't know why it was there; there might be some obscure case where this is a technically breaking change, but all tests are parsed and I hope it's fine.

Note that this doesn't give us any significant performance improvements; I already unwrap single-expression blocks during compilation.

(Which is good because otherwise I'd be worried about this change; I remember that some time ago skipping these block nodes wasn't safe).

[changeset-bot]

🦋 Changeset detected

Latest commit: 4ce0af0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@quri/squiggle-lang	Patch
@quri/squiggle-components	Patch
@quri/prettier-plugin-squiggle	Patch
@quri/versioned-squiggle-components	Patch
vscode-squiggle	Patch
@quri/squiggle-textmate-grammar	Patch

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[OAGr]

hm...

Hub preview is down. Not sure why though.

[berekuk]

Previews on specific commits don't work anymore (I mentioned this in https://quri.slack.com/archives/C06KPA4MGU9/p1711572884389439?thread_ts=1711571253.986579&cid=C06KPA4MGU9 yesterday), but previews on branches do: https://quri-hub-git-prettier-bug-quantified-uncertainty.vercel.app/

[OAGr]

Huh, interesting, thanks!

[OAGr]

Great, I can confirm that this works!