feat: Add nat type

[mark-koch]

Closes #228

[doug-q]

I don't understand what how the width of nat and int are handled.
I don't understand what is_to_u and iu_to_s are expected to do. If widths aren't changing aren't they the identity?

Approving as this matches int.

[mark-koch]

I don't understand what how the width of nat and int are handled.

Guppy currently assumes that ints are 64 bit across the board, so int_op automatically inserts a log width of 6.

I don't understand what is_to_u and iu_to_s are expected to do. If widths aren't changing aren't they the identity?

I guess they could be, but I'd prefer to error when trying to convert a negative int to nat, or when trying to convert a nat that is too large to fit into an int64.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.30686% with 13 lines in your changes missing coverage. Please review.

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Files	Patch %	Lines
guppylang/prelude/_internal.py	72.97%	10 Missing ⚠️
guppylang/checker/core.py	80.00%	2 Missing ⚠️
guppylang/tys/ty.py	96.96%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #254   +/-   ##
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  Coverage        ?   90.88%           
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  Files           ?       44           
  Lines           ?     4937           
  Branches        ?        0           
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  Hits            ?     4487           
  Misses          ?      450           
  Partials        ?        0           

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