Installation via SPM fails due to unsafe build flags

[vlas-voloshin]

When using SPM to add TOMLDecoder as a dependency to another package via Xcode, it fails with an error:

the target 'TOMLDecoder' in product 'TOMLDecoder' contains unsafe build flags

Per documentation:

As some build flags can be exploited for unsupported or malicious behavior, the use of unsafe flags makes the products containing this target ineligible for use by other packages.

The unsafe flags "-Xfrontend", "-require-explicit-sendable" have been introduced in #229. Can you please remove them, at least from the non-development definition of the package? Thanks!