Add enum class and nested enumeration example

[jahan-addison]

I could not find an enum class and nested enum example to replace a nested switch case on enums. matchit was very helpful in enumerating complicated nested cases

[BowenFu]

@jahan-addison Thanks for adding the sample. Regarding nested example, there could be two cases, 1) both values are independent, 2) the second value is dependent on how the first one is matched.

For the first case, the codes could be more concise like

return m::match(os_type, arch_type)(
    m::pattern | m:: ds(OS_Type:: Linux, Arch_Type::ARM64) = 2,
    m::pattern | m::ds(OS_Type:: Linux, Arch_Type:: X8664) = 5,
    m::pattern | m::ds(OS_Type::BSD, Arch_Type:: X8664) = 4,
    m::pattern | m::_ = 1);

For the second case, the nested version could be more valuable.

[jahan-addison]

Nice, I'll add both; in my case the values were dependent, but independent is another use case too

[BowenFu]

Thank you Jahan. I just merged it.