Use in keyword wherever possible

there is no need to indirectly create a copy of parameters in most cases. using in can drastically improve performance for several methods and operators.

for example changing float4 multiply to

    public static float4 operator * (in float4 lhs, in float4 rhs) { return new float4 (lhs.x * rhs.x, lhs.y * rhs.y, lhs.z * rhs.z, lhs.w * rhs.w); }

results in a massive performance improvement in .Net 5 (hopefully similar in Mono) ~3.1 ns vs ~0.1 ns (intel 7th gen)

burst might do that already internally but it could finally make this library faster than the built in Vector3/4/... outside of Burst

PS: soon™ https://docs.microsoft.com/en-us/dotnet/api/system.math.sincos?view=net-6.0

Start with the float4 multiplication operator shown in the issue, then inspect other methods and operators that copy parameters. Determine which cases can use the in keyword without changing behavior, and verify that the affected operations retain their behavior while improving performance outside Burst.