fix: move not resetting source location

[zerbina]

Summary

Use a MIR pass to lower the move and wasMoved magic calls into
assignments, fixing move not always resetting the source location for
both the VM and JS backends. The C backend was not affected.

Details

A x = move(y) call is now lowered into the following MIR:

x = move y
y = default()

and a wasMoved(x) call into:

x = default()

which is equivalent in behaviour to the code previously produced by
cgen -- using a shared MIR pass for the lowering ensures that the
behaviour is consistent across all backends.

The implementation of the two magics is removed from each code
generator, and a test for making sure move works correctly is added.

Previous behaviour

For the VM backend, the source location was never reset on move,
while for the JS backend, the source location was reset with
genericReset, which left record-like locations empty (reading the
fields then yield undefined) and ignores primitive types like int or
float.

[zerbina]

Scanning the body of every procedure for magics has overhead, and it'd be better to only run the pass if the relevant magics are actually used within the body.

My plan is store a set[TMagic] in MirBody, for keeping track of all magics currently present in the body. A pass for lowering, say, mWasMoved then only has to run when the magic is present in the set.

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As for mMove, it ideally should be turned into def _1 = move x; wasMoved(name x) before the move analyzer runs. At the moment, the move analyzer doesn't know that a move(x) call actually moves the value out of the location, preventing some optimizations.

For example, for:

var x = @[...]
var y = move(x)

the compiler currently injects a destructor for both x and y, even though x is guaranteed to not require destruction (the value was moved on all paths).

[saem]

/merge

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• more progress towards simplifying code generation
• a separate fix is required for the tarcmisc.nim test failure