Misleading result from `getStaticValue()` when called on mutable const-declared variables

[ninevra]

Transferring this issue from mysticatea#10 as requested in mysticatea#10 (comment).

i don't currently use this library, and haven't verified that the issue still applies; i did skim the commit log, though, and didn't see anything related.

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When getStaticValue() is called with a scope and encounters an identifier referring to a variable declared with const, it computes that variable's static value based on its const declaration initializer only. If the variable is initialized to a mutable value, it can later be modified, resulting in the return value from getStaticValue() not matching the variable's true value at time of use.

Example:

const mutable = {a: 1};
mutable.b = 2;
mutable;

Calling getStaticValue() on the Identifier node mutable on line 3 returns {value: {a: 1}}, but mutable's actual value is {a: 1, b: 2}.

This can also result in erroneously classifying identifiers as static. For example:

const mutable = {a: 1};
mutable.b = foo();
mutable;

mutable on line 3 is not static-valued, but getStaticValue() returns {value: {a: 1}}.

[voxpelli]

We should definitely add a test case to check whether this may still be the case.

Would you be up for making a PR?