Expose lookup functions through TypedProgram

[tothambrus11]

Two of the lookup functions from the type checker are exposed in the TypedProgram with non-mutating interface, so that they can be used from the HyloDoc library for name resolution of Hylo code references inside documentation.

The exposed functions are:

• unqualified lookup based on a stem identifier, inside a provided scope
• qualified lookup based on an identifier, a given type as a nominal scope and the scope of use that determines whether some conformances apply or not.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.99%. Comparing base (7e2eb24) to head (5427746).
Report is 266 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1491      +/-   ##
==========================================
+ Coverage   86.96%   86.99%   +0.02%     
==========================================
  Files         366      366              
  Lines       21752    21758       +6     
==========================================
+ Hits        18917    18928      +11     
+ Misses       2835     2830       -5     

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[dabrahams]

All newly added code should be covered by tests.

[tothambrus11]

Hi, I added tests, can you take a look again?

[tothambrus11]

Closing this PR, as the current form of the type checker is going to be replaced in the near future.