Fix: Expression with square not sorted correctly

[levirs565]

Expression with square not sorted correctly because square does not get degree 2. You can see the difference before and after this PR in table below.

Expresssion	Canocical Form Before this PR	Canocical Form After this PR	Status
x^2+x^3+x	x^{3}+x+x^2	x^{3}+x^2+x	After this PR expression sorted correctly
x^2+x^3+\sin(x)	x^{3}+x^2+\sin(x)	x^{3}+x^2+\sin(x)	Sorted correctly.
\sin(x)+x^2+x^3	x^{3}+\sin(x)+x^2	x^{3}+x^2+\sin(x)	This expression's cannocial form must equal with above expression's canonical form

Some test's expected result have been changed. You can see changed test in this table.

Expresssion	Canocical Form Before this PR	Canocical Form After this PR
x^2y^3+x^3y^2+xy^4+x^4y+x^2y^2	x^{4}y+xy^{4}+x^{3}y^2+x^2y^{3}+x^2y^2	x^{4}y+x^{3}y^2+x^2y^{3}+xy^{4}+x^2y^2
(b^3b^2)+(a^3a^2)+(b^6)+(a^5b)+(a^5)	a^{5}b+b^{6}+a^{5}+a^2a^{3}+b^2b^{3}	a^{5}b+b^{6}+a^2a^{3}+a^{5}+b^2b^{3}

Why I change the test's expected result?. Canocial form of those expression are inconsistent with other expressions that does not have square. You can see this table:

Expresssion	Canocical Form Before this PR	Canocical Form After this PR	Status
x^2y^3+x^3y^2+xy^4+x^4y+x^2y^2	x^{4}y+xy^{4}+x^{3}y^2+x^2y^{3}+x^2y^2	x^{4}y+x^{3}y^2+x^2y^{3}+xy^{4}+x^2y^2	Different
(b^3b^2)+(a^3a^2)+(b^6)+(a^5b)+(a^5)	a^{5}b+b^{6}+a^{5}+a^2a^{3}+b^2b^{3}	a^{5}b+b^{6}+a^2a^{3}+a^{5}+b^2b^{3}	Different
(b^3b^3)+(a^3a^3)+(b^6)+(a^5b)+(a^5)	a^{3}a^{3}+a^{5}b+b^{3}b^{3}+b^{6}+a^{5}	a^{3}a^{3}+a^{5}b+b^{3}b^{3}+b^{6}+a^{5}`	Equal

Also, this change fix some sorting result when there are square in expression and sin function.

[arnog]

Thank you for investigating this issue and submitting this PR.
The Compute Engine codebase is undergoing a refactor which unfortunately makes merging this PR directly a little bit tricky. However, I will make sure to incorporate your changes and preserve their effect.
Again, thanks for your submission, it is very much appreciated.