Minor adjustments to enable use in alloc environments without std.

[pperanich]

These changes enable the async feature to be used in no_std when alloc is still available. All tests still passed.

• Use hashbrown::HashMap instead of std::collections::HashMap for alloc compatibility.
• Remove standard library dependency from the async feature, enabling use in no_std environments when alloc present.
• Replace std::boxed::Box with alloc::boxed::Box in future types to support environments without the complete std library.