fix: add duplicate name check in Substance checker

[wodeni]

Description

Related issue/PR: #654

The Substance checker doesn't check for duplicated names (since the Haskell days!?). This PR adds duplicate name checks for Decls.

Implementation strategy and design decisions

• In checkStmt, check if the name exists right after the type constructor check.
• Fix all tests in the existing suite that had duplicate names (surprise!)
• Add a test of the example in Substance checker not catching name collisions across types #654 in the suite.

[codecov]

Codecov Report

Merging #657 (69e6b7e) into main (a50ccae) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #657      +/-   ##
==========================================
+ Coverage   66.54%   66.56%   +0.01%     
==========================================
  Files          44       44              
  Lines        7106     7109       +3     
  Branches     1337     1338       +1     
==========================================
+ Hits         4729     4732       +3     
  Misses       2368     2368              
  Partials        9        9              

Impacted Files	Coverage Δ
packages/core/src/index.ts	77.51% <ø> (-0.18%)	⬇️
packages/core/src/compiler/Substance.ts	90.47% <100.00%> (+0.12%)	⬆️

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[samestep]

lgtm other than a couple random questions