Fix bug in Gröbner basis caching

[wegank]

In the following test:

AlgebraicSolving.jl/test/algorithms/groebner-bases.jl

Lines 22 to 34 in f22e334

	I = Ideal([x+2*y+2*z-1, x^2+2*y^2+2*z^2-x, 2*x*y+2*y*z-y])
	G = groebner_basis(I, eliminate=2, intersect=false)
	H = MPolyRingElem[
	z^4 + 38*z^3 + 95*z^2 + 95*z
	30*z^3 + 32*z^2 + y + 87*z
	41*z^3 + 37*z^2 + x + 30*z + 100
	]
	@test G == H
	@test_throws ErrorException eliminate(I,0)
	L = eliminate(I,2)
	@test L == H
	@test I.gb[2] == H

Line 23 and Line 32 unexpectedly give the same results due to #135. This PR fixes the issue while maintaining maximum compatibility, by using truncated hashes as negative Int64 dictionary keys.

Closes #135.