Fix assembly backend

[Pratyush]

Description

Fix assembly backend to actually reduce the result of squaring/multiplication. Depending on the available CI runner, also runs tests with the assembly backend.

The bug arises because the assembly backend (before this PR) did something like the following:

fn square_in_place(&mut self) {
	let mut a = (self.0).0; // `a` is a *copy* of the data in `self`.
	// do assembly stuff with `a`
	// this is where the bug arose: all assembly operations happened on `a`, 
	// but we didn't write the result back to `self`.
}

In this PR, we fix the above code so that operations do indeed happen in place.

fn square_in_place(&mut self) {
	let a = &mut (self.0).0; // Now `a` is a mutable reference to the contents of `self`.
	// do assembly stuff with `a`
}

---

Before we can merge this PR, please make sure that all the following items have been
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• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[weikengchen]

Looks good.

[jon-chuang]

Possible alternate fix: ctx.add_declaration("a", "r", "a"); -> ctx.add_declaration("a", "r", "&mut a");

Not sure why this was changed but glad it's fixed