Compile for some embedded targets fails

[stepkun]

Hi,

thanks four your great work.

When using your crate with some embedded targets I get the following error:

error[E0433]: cannot find sync in alloc
--> ../arcstr-1.2.0/src/arc_str.rs:1381:30
|
1381 | impl From for alloc::sync::Arc {
| ^^^^ could not find sync in alloc
|
note: found an item that was configured out
--> ../.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/lib.rs:234:9
|
233 | #[cfg(all(not(no_rc), not(no_sync), target_has_atomic = "ptr"))]
| ------------------------- the item is gated behind the ptr feature
234 | pub mod sync;
| ^^^^

Some popular embedded targets do not support the atomics needed for alloc::sync::Arc (pi pico, esp32-c3).
So to compile for those targets the alloc::sync::Arc implementations should be gated behind the same ptr feature.

Kind regards
Stephan