Add FAST GetAttribute methods.

[soi013]

Hi,

This PR is adding fast GetAttribute methods.

How to use

//from Member 
Fruits.Apple.ToMember().GetAttribute<DescriptionAttribute>()?.Description;

//Direct: it is little bit faster, but verbose(`<Fruits`).
Fruits.Apple.GetAttribute<Fruits, DescriptionAttribute>()?.Description;

Benchmark

Method	Mean	Error	StdDev	Ratio
NetCore	1,308.74 ns	119.295 ns	6.5390 ns	1.000
EnumsNet	24.24 ns	2.391 ns	0.1310 ns	0.019
FastEnumFromMember	17.09 ns	2.669 ns	0.1463 ns	0.013
FastEnumDirect	11.42 ns	2.344 ns	0.1285 ns	0.009

[xin9le]

Thank you for your PR! I like your attribute caching approch. However, I have two questions.

Questions

1. There is a possibility of two or more attribute annotated (like LabelAttribute). I think your PR doesn't implement its case.

2. What attributes do you want to annotate to the enum field? Almost case, I think LabelAttribute and EnumMemberAttribute are enough.

[soi013]

There is a possibility of two or more attribute annotated (like LabelAttribute). I think your PR doesn't implement its case.

OK. I will add it.

What attributes do you want to annotate to the enum field? Almost case, I think LabelAttribute and EnumMemberAttribute are enough.

I usually use original attributes for extend enum.
It's a sample.

enum FileTypes
{
    [FileExt(".txt"), Description("Full text"), CanRead, CanWrite]
    FullText,
    [FileExt(".txt"), Description("Simple text: can't read"), CanWrite]
    SimpleText,
    [FileExt(".mybin2"), Description("my original format"), CanRead, CanWrite]
    Binary,
    [FileExt(".mybin"), Description("my old original format"), CanRead, CanWrite, Obsolete]
    OldBinary,
}

//At file save dialog
FastEnum.GetValues<FileTypes>()
    .Where(x => x.GetAttribute<FileTypes, CanWrite>() != null)
    .Where(x => x.GetAttribute<FileTypes, Obsolete>()==null)
    .Select(x =>
        (
            x.GetAttribute<FileTypes, FileExtAttribute>().FileExt,
            x.GetAttribute<FileTypes, DescriptionAttribute>().Description
        ))
    .Select((a) =>$"{a.Description} (*{a.FileExt})|*{a.FileExt}")
//create Filter of dialg

[soi013]

I added GetAttribute"s" methods.

This is Benchmark.

Method	Mean	Error	StdDev	Ratio
NetCore	2,098.96 ns	33.497 ns	1.8361 ns	1.00
EnumsNet	177.24 ns	2.348 ns	0.1287 ns	0.08
FastEnumFromMember	70.98 ns	11.527 ns	0.6318 ns	0.03
FastEnumDirect	79.32 ns	299.535 ns	16.4185 ns	0.04

[xin9le]

I think almost ok. However I'm considering about a better way now. Please wait for a while.