Improve external def parsing

[johnynek]

close #1170

This does a few things:

1. allows external def to have type parameters. This allows setting the kinds on the type parameters other than kind *.
2. error if the type parameters do not exactly match the free types in the external def.
3. verify that the final type of the external def is well kinded. Previously, we just took parsed syntax and accepted whatever was claimed, but that allowed to make values of the wrong kind.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.07843% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 91.49%. Comparing base (04a0e87) to head (20ce57e).
Report is 20 commits behind head on main.

❗ Current head 20ce57e differs from pull request most recent head 01f3832. Consider uploading reports for the commit 01f3832 to get more accurate results

Files	Patch %	Lines
.../main/scala/org/bykn/bosatsu/SourceConverter.scala	96.42%	1 Missing ⚠️
...re/src/main/scala/org/bykn/bosatsu/Statement.scala	90.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1188      +/-   ##
==========================================
+ Coverage   91.32%   91.49%   +0.17%     
==========================================
  Files          96       96              
  Lines       11846    12047     +201     
  Branches     2675     2763      +88     
==========================================
+ Hits        10818    11023     +205     
+ Misses       1028     1024       -4     

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