Ensure FilterMap returns a concrete type

[BooleanCat]

Please provide a brief description of the change.

Ensure FilterMap returns a concrete type. It erroneously returns an interface at the moment.

Which issue does this change relate to?

None.

Contribution checklist.

• [x] I have read and understood the CONTRIBUTING guidelines
• [x] My code is formatted (make check)
• [x] I have run tests (make test)
• [x] All commits in my PR conform to the commit hygiene section
• [x] I have added relevant tests
• [x] I have not added any dependencies

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3833758) 100.00% compared to head (36ce292) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #56 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 18 18 Lines 382 382 ========================================= Hits 382 382 ``` | [Files Changed](https://app.codecov.io/gh/BooleanCat/go-functional/pull/56?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tom+Godkin) | Coverage Δ | | |---|---|---| | [iter/lift.go](https://app.codecov.io/gh/BooleanCat/go-functional/pull/56?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tom+Godkin#diff-aXRlci9saWZ0Lmdv) | `100.00% <ø> (ø)` | | | [iter/filter.go](https://app.codecov.io/gh/BooleanCat/go-functional/pull/56?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tom+Godkin#diff-aXRlci9maWx0ZXIuZ28=) | `100.00% <100.00%> (ø)` | |

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