Optimise `get_krylov_equations`

[oameye]

Optimise get_krylov_equations. Not a priority as it is only runs once in a generic workflow.

using HarmonicBalance, BenchmarkTools
@variables α ω ω0 F γ t x(t);
diff_eq = DifferentialEquation(d(x, t, 2) + ω0^2*x + α * x^3 + γ * d(x, t) ~ F*cos(ω*t), x)
add_harmonic!(diff_eq, x, ω) #
@btime harmonic_eq = get_harmonic_equations(diff_eq)
@btime krylov_eq = get_krylov_equations(diff_eq, order=1)
@btime krylov_eq = get_krylov_equations(diff_eq, order=2)

106.079 ms (727806 allocations: 35.85 MiB)
252.987 ms (2011451 allocations: 105.51 MiB)
2.251 s (17602636 allocations: 953.39 MiB)