Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

First try `jobserver::Client::try_acquire`, which will work:
 - If a fifo is used as jobserver
 - On linux and:
    - preadv2 with non-blocking read available (>=5.6)
    - /proc is available
 - On Windows
 - On wasm

if not, we will simply fallback to help thread implementation, spawning one
thread to maintain compatibility with other platforms.

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

Also impls `Send`, `Sync`, `RefUnwindSafe` and `UnwindSafed` when the `T`
meets the criterior.

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

whenever feature parallel is enabled.

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

There is no need to, it never fails.

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>