Generate SyntaxError of global declaration

[HyeockJinKim]

Generate SyntaxError instead of SyntaxWarning
if global declaration is for priviously used variable

Fixes#72

[corona10]

@HyeockJinKim
Raising SyntaxError is the right solution but
It looks like tests should be updated

[corona10]

@ncw
Would you like to guide about updating the unit test for him?
If his approach is not proper, feedback will be helpful to him

[codecov-io]

Codecov Report

Merging #74 into master will not change coverage.
The diff coverage is 100%.

@@ Coverage Diff @@## master #74 +/- ##
=======================================
Coverage 68.65% 68.65% =======================================
Files 59 59 Lines 10525 10525 =======================================
Hits 7226 7226 Misses 2790 2790 Partials 509 509

Impacted Files	Coverage Δ
symtable/symtable.go	94.23% <100%> (ø)	⬆️

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[HyeockJinKim]

@corona10 Can I get a review?

[corona10]

@HyeockJinKim
It looks working well but I am not sure this is the right way to deal with.
Let's wait @ncw 's review

[ncw]

I think the code looks nice :-)

However it makes gpython incompatible with python3.4 as I wrote here: #72 (comment)

Now I don't think any of the tests rely on that so it is safe to change it.

What do you think @corona10 - make a break with python3.4 compatibility here? On balance I think it is a good idea since this is almost always an actual problem.

PS the commits need squashing which can be done at merge time

[corona10]

@ncw
I am not on +1 or -1 on breaking the 3.4 compatibilities.
However when we update the project objective into implementing Python3.x interpreter.
Breaking with python3.4 compatibility is not a bad idea if so that's a way we have to go.

[ncw]

@ncw
I am not on +1 or -1 on breaking the 3.4 compatibilities.
However when we update the project objective into implementing Python3.x interpreter.
Breaking with python3.4 compatibility is not a bad idea if so that's a way we have to go.

OK, lets squash and merge :-)

[corona10]

@HyeockJinKim
Thanks for the contribution
and thanks for the review @ncw :)