Add SPI imports for SwiftCorelibsFoundation usage of Decimal (#751)

* Add SPI imports for SwiftCorelibsFoundation usage of Decimal

Also adds some tests for the exported functionality that SCL-F uses.

* Rename and rearrange some internal vs SPI methods

* Remove obsolete comment

Author: parkera

Sources/FoundationEssentials/Decimal/CMakeLists.txt

@@ -14,4 +14,5 @@
 target_sources(FoundationEssentials PRIVATE
     Decimal.swift
     Decimal+Conformances.swift
-    Decimal+Math.swift)
+    Decimal+Math.swift
+    Decimal+Compatibility.swift)