Allow linker to perform deadcode elimination for programs using kingpin

[rosstimothy]

This was largely inspired by spf13/cobra#1956.

The usage rendering relied on text/template which in turn relied on reflect.MethodByName. As a result, Go's deadcode elimination is prevented from running, and thus means any downstream consumers of kingpin also cannot take advantage of deadcode elimination.

This changes the usage rendering of kingpin such that text/template is not used by default. The existing templates were converted into pure Go functions, and an extensive test suite was added to ensure equality with the legacy templates. Default applications now use the pure Go equivalent of kingpin.DefaultUsageTemplate. The existing UsageTemplate and UsageFuncs APIs remain intact to preserve compatibility. Any use of either API will result in text/template rendering usage and preventing deadcode elimination.

The basic app used in TestDeadCodeElimination was tested before and after this change. The usage text remained the same, but the binary size shrunk from ~5MB to ~2MB.

[rosstimothy]

@alecthomas I know this is a larger change but I would really appreciate your review. Being able to leverage DCE can result in a tremendous binary size reduction for consumers of kingpin. I've seen ~40% reduction in larger projects. Datadog published https://www.datadoghq.com/blog/engineering/agent-go-binaries where they saw >70% reduction in binary sizes with DCE.

[alecthomas]

Seems reasonable to me, thanks!