fix duplicate key deletion when forget called (#14)

Check for error in TestDo_panic

Even the panic shoud prevent for reaching the error value
the error is checked for more visibility and linter warnings

Protect the shared value for concurrent access

Preserve context values (#6)

Instead of passing a fresh `context.Background()` wrap context passed by
the user with a context that ignores cancellation.

This allows to drop the original cancellation while preserving context
values.

If built with Go 1.21+ the stdlib `context.WithoutCancel` is used.
Otherwise it fallbacks to an in-tree copy of the withoutCancel.

Signed-off-by: Paweł Gronowski <[email protected]>

Ensure no function calls after one context is cancelled

Update github workflow

Use go 1.18 in GitHub workflow

Pass a context to the calling function for cancellation

Initial commit