Improve inference for recursive functions

[codecov-commenter]

Codecov Report

Patch coverage: 87.50% and project coverage change: -0.03% :warning:

Comparison is base (dc1c4c2) 91.79% compared to head (c025b9d) 91.77%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1030 +/- ## ========================================== - Coverage 91.79% 91.77% -0.03% ========================================== Files 91 91 Lines 9449 9460 +11 Branches 2206 2208 +2 ========================================== + Hits 8674 8682 +8 - Misses 775 778 +3 ``` | [Files Changed](https://app.codecov.io/gh/johnynek/bosatsu/pull/1030?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P.+Oscar+Boykin) | Coverage Δ | | |---|---|---| | [.../src/main/scala/org/bykn/bosatsu/rankn/Infer.scala](https://app.codecov.io/gh/johnynek/bosatsu/pull/1030?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P.+Oscar+Boykin#diff-Y29yZS9zcmMvbWFpbi9zY2FsYS9vcmcvYnlrbi9ib3NhdHN1L3JhbmtuL0luZmVyLnNjYWxh) | `95.63% <86.36%> (-0.56%)` | :arrow_down: | | [core/src/main/scala/org/bykn/bosatsu/Region.scala](https://app.codecov.io/gh/johnynek/bosatsu/pull/1030?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P.+Oscar+Boykin#diff-Y29yZS9zcmMvbWFpbi9zY2FsYS9vcmcvYnlrbi9ib3NhdHN1L1JlZ2lvbi5zY2FsYQ==) | `100.00% <100.00%> (ø)` | | ... and [6 files with indirect coverage changes](https://app.codecov.io/gh/johnynek/bosatsu/pull/1030/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P.+Oscar+Boykin)

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