var-get should throw a runtime error if variable does not exist

[csgui]

For now, var-get on an non-existing variable could lead to the generation of invalid Wasm code.

To remedy this, there is an "easy enough" fix to write:

1. Change the linked function link_get_variable_fn to write an optional Value to the memory;
2. Adapt the traverse function of get-var to unwrap the optional, and return a runtime error if it's none.

[Acaccia]

@csgui found something AMAZING in link_get_variable_fn: the type of the returned variable varies depending on if the var exists or not. If it exists, the type is the type of the var. If not, it's an optional.

So in this case, we will need a horrible workaround, where we will have to check if the fetched variable has the same type as the computed return type:

• if yes, we have to wrap both the var and the type in Optional to make it coherent,
• if not, we have to change only the return type to Optional.

Actually,if not, we could just write a "None" on the first byte which will contain the result, and in the wasm function check the first byte to see if we runtime-error or not.