async-coap-uri: no_std implementation is broken

[Luro02]

The crate provides the following features:

[features]
default = ["std"]
std = ["alloc"]
alloc = []

by default std is enabled, which works as expected.

To test the features I simply ran the tests of async-coap-uri with the below commands.

To disable all default features one could try the following command (this would be true no_std without any allocator):

cargo test --no-default-features

which fails (with lots of errors).

Enabling only the alloc-feature also fails

cargo test --no-default-features --features alloc

[darconeous]

Yep, this is a bug. Architecturally, rust-async-coap is designed to eventually allow for use in no_std mode without even alloc. However, getting things to work without alloc is going to require generic associated types to land, so I didn't bother too much with it other than to make sure that architecturally it wasn't precluded from being introduced later.

However, there is no good reason for not supporting no_std with alloc right now. Getting that implemented would be a good step toward reaching that eventual goal once GAT lands.