deprecation: add runtime warnings to deprecated exec/which methods

[silug]

Short description

Add explicit runtime deprecation warnings (via Facter.warnonce) to methods that are slated for removal in a future major version:

• Facter::Core::Execution.exec: warn callers to use .execute instead
• Facter::Util::Resolution.exec: replaced bare public :exec with an explicit method that warns and delegates to Facter::Core::Execution
• Facter::Util::Resolution.which: same treatment as .exec above

Specs updated to assert the deprecation warning is emitted; new specs added for the Resolution class-level deprecated methods.

References #88, #89

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)