[minor] more DRY importc with `{.importc: “freeimage_$1”.}`
when wrapping libs it'll be common that each symbol is prefixed / postfixed by a fixed name eg freeimage; in this case user may want to remove all such common prefix/suffix in wrapper, eg:
```
proc foo(){.importc: “freeimage_foo” .}
proc bar(){.importc: “freeimage_bar” .}
```

## proposal
we can make resulting wrappers more DRY:
```
proc foo(){.importc: “freeimage_$1” .}
proc bar(){.importc: “freeimage_$1” .}
```

main benefit: 
It’s not just for DRY, it’s for human eye to focus on parts that require his attention (the occasial violation caused by a user-issued or nimterop issued custom rename) as opposed to the predictable (`freeimage_foo` => `foo`)

## implementation:
trivial, just find whether nimName is a prefix or suffix of origName in genImportC, see https://github.com/genotrance/nimterop/pull/88/files
