Add std flag

[heytdep]

What

Adds a new feature and disables allocator when std flag is set.

Why

Allows to use the std while having a custom allocator.

Known limitations

N/A

[dmkozh]

This can be breaking for someone, can't it? No-one would have the new feature enabled, so handle_panic may disappear suddenly. Wouldn't it be safer to add a feature that disables handle_panic on-demand?

[leighmcculloch]

There's a variety of situations that we could support, and so I'm not sure using 'std' as the name of this feature is most appropriate because of the possible variety of interpretations.

Here's a matrix of configurations that could be supported. Only the two configurations with text in there are the ones currently supported. The text in the boxes is the feature enabled to get those things. Today we have:

	no allocator	sdks allocator	custom allocator	libstd allocator
sdk panic handler	default	alloc
custom panic handler
libstd panic handler

If this PR is merged, the result would be support for:

	no allocator	sdks allocator	custom allocator	libstd allocator
sdk panic handler	default	alloc
custom panic handler	std
libstd panic handler

If we wanted to support all these things, we could add the following features:

	no allocator	sdks allocator	custom allocator	libstd allocator
sdk panic handler	default	alloc	alloc-custom	alloc-std
custom panic handler	panic-handler-custom
libstd panic handler	panic-handler-std

To that end, I think this PR's new feature could be named panic-handler-custom, because the feature is disabling the built-in panic handler, and leaving it up to the developer to provide the panic handler in some way, either by importing a lib that contains one like libstd, or providing their own.