New option `-sort` to auto generate sorting methods for struct

Auto-generated code has a new option to generate sorting methods for struct:

-sort "Employee:Name, Employee:Salary"
eg:
//go:generate gofp -destination fp.go -pkg employee -type "Employee" -sort "Employee:Name"
type Employee struct {
Id int
Name string
Salary float64
}

Generated sorting functions:

1. SortEmployeeByName
2. SortEmployeeByNameDesc
3. SortEmployeeByNamePtr
4. SortEmployeeByNameDescPtr

New Standard functions:

Reverse: To reverse list
Ex. ReverseInt
Keys: return keys of the map
Take: return n items from the list