Multi level debug support

[PhilDay-CT]

I was recently hitting an issue when using the network stack of running out of memory, but I couldn't see that was the issue because the debug output in the network stack is disabled by default. Enabling it seems to also be expensive in memory terms, so it just shifted the issue. It would be nice / helpful to be able to enable only error messages. We could do this with the current code with something like:

using Debug = ConditionalDebug<false, "TLS">;
using Warn = ConditionalDebug<false, "TLS">;
using Error = ConditionalDebug<true, "TLS">;
...

but that feels kind of clumsy.   Could we instead have something that gives the following, conditionally enabled, methods:

```c++
   Debug::log()
   Debug::warn()
   Debug::error()

The driver_adaptor code has it's own workaround for an unmaskable message with:

    if (faultInjected)
    {
        ConditionalDebug<true, "Ethernet Adaptor">::log("Triggering crash");
				  

[davidchisnall]

I think this is a good idea. I've wanted to replace that bool with a severity level enum for a while, then add a severity template argument to log, Assert, and Invariant with a default of Info. Patches very welcome for this!

[davidchisnall]

Ideally we'd also make the debug options in xmake into something that takes a string, maybe one of:

• None / n
• Error
• Warn
• Info / y

That would keep the old version working for the cases with the option. I think we'd need to replace the existing template argument with a small wrapper that can be constructed from an enum or a bool, where the bool maps to the lowest / highest values in the enum.