Regression In instance_eval

[fundamental]

It looks like while fixing issue #3072 at commit fb2e0b7 a regression was introduced to instance_eval.

Test case:

class Foo
    def bar(x)
        instance_eval "def baz; #{x}; end"
    end
end

f1 = Foo.new
f2 = Foo.new
f1.bar 1
f2.bar 2
puts "f1.baz = #{f1.baz} (expected 1)"
puts "f2.baz = #{f2.baz} (expected 2)"

CRuby output:

f1.baz = 1 (expected 1)
f2.baz = 2 (expected 2)

mruby output:

f1.baz = 2 (expected 1)
f2.baz = 2 (expected 2)

Using mrb_run instead of mrb_top_run appears to fix the incorrect behavior demonstrated by the test case, though I assume a somewhat more complex solution may be needed to produce the correct behavior for both this issue and #3072.