remove custom opam version comparison which is inconsistent with opam, fix #149

instead of merging min and max constraints, collect them in a set and output all of them, leaving the merge to opam

i.e. consider the collected bounds:

package ~min:"1.0" ~max:"2.0" "a"
package ~min:"1.2" ~max:"3.0" "a"
package ~min:"1.5" ~max:"2.0" "a"

it used to merge them, and produce "a" {>= "1.5" & < "2.0"}

now, it'll output "a" {>= "1.0" & >= "1.2" & >= "1.5" & < "2.0" & < "3.0"}

the previous approach failed on various version identifiers that are valid in opam, such as "v1.0", "1.2.3-99", "2.0~beta" -- since the functoria version parser only allowed dot "." and numbers.

mirage / functoria

remove custom opam version comparison which is inconsistent with opam, fix #149 #183