run breaks type safety

[byorgey]

Describe the bug
Executing run can overwrite existing definitions with new definitions having different types. This can break type safety if the overwritten names were used in the same expression.

To Reproduce
Create a file m.sw:

echo "def m = move end" > m.sw

Now, in a Creative mode game:

> def m = 4 end; run "m.sw"; return (m + 1)

Results in a Fatal error: bad application of execConst...

Expected behavior
I don't even know what is expected. This is just yet more evidence that run is broken; see #495 . This would not be a problem with a proper import command since the second m would be typechecked with the type it has in m.sw, as it should.

Additional context
I thought of this when reading @xsebek's clever hacks in #2064 😄

[xsebek]

Well, run should probably not let its definitions escape, but then it would not be as useful.

I am surprised that this bug allows me to use m. It does feel a bit like calling external functions from C - if you mess up the signature, you get a runtime error.

[byorgey]

Well, run should probably not let its definitions escape

That is the entire point of run 😆

[xsebek]

The name does sound like it just runs a script with commands - it even has type Cmd Unit.

If import was limited to importing definitions, I would be OK with run just running scripts and not overwriting outer scope definitions.