Implement built-ins

[praiskup]

TODO list, see https://rpm.org/docs/4.20.x/manual/macros
Please double-check that no macro we implement here has dangerous side effects.

• %gsub
• %lower/%upper
• %len
• %basename/%dirname
• %expand
• %dnl
• %expr
• %reverse
• %shrink
• %suffix
• %rep
• %url2path
• %shescape
• %uncompress
• ...