Fix *vm.VM memory leak

[diegommm]

Currently, it's reslicing using [0:0] (which is equivalent to [:0]), but this does not clear the memory references in the underlying array, which prevents garbage collection. For instance, if:

• A VM runs a program and pushes N items to the stack
• The last item is (or contains) a pointer or reference type
• The VM is later reused, but it always pushes less than N items to the stack

Then the Nth old item will not be garbage collected because there is still a reference to it in the underlying array.

So we need to "clear the tail" of the slice before reslicing to avoid this. See these example issues:

• https://go.dev/blog/slices-intro#a-possible-gotcha
• https://go.dev/blog/generic-slice-functions#the-fix

[antonmedv]

Slightly slower bench, but this is expected.

_expr_reuseVm-4               70.73n ± 1%   75.88n ± 1%   +7.28% (p=0.001 n=10)

Reusing VM still faster than creating stack vm each time:

_expr-4                       114.2n ± 0%   119.9n ± 3%   +4.99% (p=0.000 n=10)
_expr_reuseVm-4               70.73n ± 1%   75.88n ± 1%   +7.28% (p=0.001 n=10)