Add flatten_variable

[blegat]

Part of https://github.com/JuliaAlgebra/MultivariatePolynomials.jl/issues/194

Int64

Benchmark 0

Before

	Time	Alloc	Memory
SIMDPolynomials	376.346 ns	8	400 bytes
DynamicPolynomials	6.153 μs	196	10.81 KiB
TypedPolynomials	165.017 ns	4	336 bytes

After

	Time	Alloc	Memory
SIMDPolynomials	358.810 ns	8	400 bytes
DynamicPolynomials	6.579 μs	196	10.81 KiB
TypedPolynomials	170.237 ns	4	336 bytes

Benchmark 1

Before

	Time	Alloc	Memory
SIMDPolynomials	129.194 μs	1904	2.10 MiB
DynamicPolynomials	1.067 ms	24046	3.28 MiB
TypedPolynomials	118.609 μs	1125	2.25 MiB

After

	Time	Alloc	Memory
SIMDPolynomials	132.809 μs	1904	2.10 MiB
DynamicPolynomials	1.166 ms	24080	3.28 MiB
TypedPolynomials	124.154 μs	1125	2.25 MiB

Benchmark 2

Before

	Time	Alloc	Memory
SIMDPolynomials	4.766 μs	266	44.00 KiB
DynamicPolynomials	53.152 μs	1983	161.02 KiB
TypedPolynomials	5.725 μs	191	74.66 KiB

After

	Time	Alloc	Memory
SIMDPolynomials	4.770 μs	263	43.88 KiB
DynamicPolynomials	55.238 μs	1967	160.36 KiB
TypedPolynomials	5.576 μs	188	74.64 KiB

Benchmark 3

Before

	Time	Alloc	Memory
SIMDPolynomials	690.351 μs	31375	17.62 MiB
DynamicPolynomials	5.755 ms	156579	25.83 MiB
TypedPolynomials	378.289 μs	6438	25.03 MiB

After

	Time	Alloc	Memory
SIMDPolynomials	677.469 μs	31351	17.62 MiB
DynamicPolynomials	6.198 ms	156645	25.84 MiB
TypedPolynomials	372.314 μs	6414	25.02 MiB

BigInt

Benchmark 0

Before

	Time	Alloc	Memory
SIMDPolynomials	908.128 ns	42	1008 bytes
DynamicPolynomials	6.748 μs	236	11.50 KiB
TypedPolynomials	716.375 ns	38	944 bytes

After

	Time	Alloc	Memory
SIMDPolynomials	931.929 ns	42	1008 bytes
DynamicPolynomials	6.983 μs	236	11.50 KiB
TypedPolynomials	724.700 ns	38	944 bytes

Benchmark 1

Before

	Time	Alloc	Memory
SIMDPolynomials	424.879 μs	14864	1.93 MiB
DynamicPolynomials	1.139 ms	35376	3.06 MiB
TypedPolynomials	433.056 μs	13983	2.08 MiB

After

	Time	Alloc	Memory
SIMDPolynomials	438.317 μs	14864	1.93 MiB
DynamicPolynomials	1.216 ms	35406	3.07 MiB
TypedPolynomials	430.715 μs	13983	2.08 MiB

Benchmark 2

Before

	Time	Alloc	Memory
SIMDPolynomials	9.612 μs	422	46.88 KiB
DynamicPolynomials	58.479 μs	2187	165.34 KiB
TypedPolynomials	12.317 μs	314	76.76 KiB

After

	Time	Alloc	Memory
SIMDPolynomials	9.882 μs	419	46.76 KiB
DynamicPolynomials	61.901 μs	2167	164.78 KiB
TypedPolynomials	12.040 μs	311	76.74 KiB

Benchmark 3

Before

	Time	Alloc	Memory
SIMDPolynomials	2.589 ms	82097	18.48 MiB
DynamicPolynomials	7.164 ms	202267	26.71 MiB
TypedPolynomials	3.188 ms	57112	25.88 MiB

After

	Time	Alloc	Memory
SIMDPolynomials	2.633 ms	82073	18.48 MiB
DynamicPolynomials	7.521 ms	202263	26.72 MiB
TypedPolynomials	3.141 ms	57088	25.88 MiB

Rational{BigInt}

Benchmark 0

Before

	Time	Alloc	Memory
SIMDPolynomials	2.933 μs	225	5.48 KiB
DynamicPolynomials	7.901 μs	390	15.22 KiB
TypedPolynomials	2.741 μs	221	5.42 KiB

After

	Time	Alloc	Memory
SIMDPolynomials	3.026 μs	225	5.48 KiB
DynamicPolynomials	8.217 μs	390	15.22 KiB
TypedPolynomials	2.861 μs	221	5.42 KiB

Benchmark 1

Before

	Time	Alloc	Memory
SIMDPolynomials	1.592 ms	76435	4.21 MiB
DynamicPolynomials	2.240 ms	89947	5.31 MiB
TypedPolynomials	1.604 ms	75715	4.37 MiB

After

	Time	Alloc	Memory
SIMDPolynomials	1.673 ms	76435	4.21 MiB
DynamicPolynomials	2.346 ms	89963	5.32 MiB
TypedPolynomials	1.679 ms	75715	4.37 MiB

Benchmark 2

Before

	Time	Alloc	Memory
SIMDPolynomials	18.604 μs	1009	79.30 KiB
DynamicPolynomials	70.529 μs	2888	201.93 KiB
TypedPolynomials	20.573 μs	897	109.32 KiB

After

	Time	Alloc	Memory
SIMDPolynomials	18.776 μs	1006	79.23 KiB
DynamicPolynomials	75.952 μs	2847	200.82 KiB
TypedPolynomials	20.142 μs	894	109.32 KiB

Benchmark 3

Before

	Time	Alloc	Memory
SIMDPolynomials	6.390 ms	242294	31.12 MiB
DynamicPolynomials	11.422 ms	339474	38.81 MiB
TypedPolynomials	7.774 ms	215247	38.58 MiB

After

	Time	Alloc	Memory
SIMDPolynomials	6.140 ms	242270	31.12 MiB
DynamicPolynomials	12.206 ms	339036	38.81 MiB
TypedPolynomials	7.747 ms	215223	38.57 MiB