Add strcase package

[lavoiesl]

• ToPascalCase, ToCamelCase, ToSnakeCase, and ToHeaderField will transform any input to that form.
• Support for unicode runes
• Support for all-uppercase initialisms, like mandated by the Go convention.
• Expose IsInitialism
• Expose SplitJoin, which can be used for any style of joins.
• Emphasis on reducing allocations for memory efficiency.