Type inference module. PR #411 part 3/3

[collinc97]

Part 3/3 for merging PR #411

Motivation

The type inference module resolves implicit types in a Leo program.
For a visual aid see https://github.com/AleoHQ/leo-semantics/blob/master/acl2/variable-environments.pdf

Each function in a Leo program is represented by a frame. Frames can store one or more type assertions which define an equality between a type and an expression. A new type assertion is created from each statement inside of a function. If a statement contains an unknown type, then a unique type variable is created for the type assertion.

After parsing all statements in a function, the frame's type assertions can be checked. Each unknown type variable is substituted for its expression type until all types are known. If any unknown types exist at the end of substitution, then the check fails indicating that the compiler cannot infer the type.

Test Plan

Unit tests have been added in the type inference module to test basic parsing and simple type inference errors.

Integration tests have been added in the compiler module to test generic leo program files and advanced type inference errors.

[codecov]

Codecov Report

Merging #439 (dafaa66) into master (0bc0ee4) will increase coverage by 1.11%.
The diff coverage is 86.03%.

@@            Coverage Diff             @@
##           master     #439      +/-   ##
==========================================
+ Coverage   72.19%   73.31%   +1.11%     
==========================================
  Files         430      453      +23     
  Lines       12267    13467    +1200     
==========================================
+ Hits         8856     9873    +1017     
- Misses       3411     3594     +183     

Impacted Files	Coverage Δ
compiler/src/errors/compiler.rs	17.14% <0.00%> (-1.61%)	⬇️
type-inference/src/errors/scope.rs	0.00% <0.00%> (ø)
type-inference/src/errors/variable_table.rs	0.00% <0.00%> (ø)
type-inference/src/errors/type_inference.rs	10.00% <10.00%> (ø)
type-inference/src/errors/type_assertion.rs	33.33% <33.33%> (ø)
type-inference/src/assertions/type_membership.rs	65.00% <65.00%> (ø)
type-inference/src/errors/frame.rs	71.57% <71.57%> (ø)
compiler/tests/syntax/mod.rs	53.33% <83.33%> (+7.50%)	⬆️
type-inference/src/objects/frame.rs	89.47% <89.47%> (ø)
type-inference/src/assertions/type_assertion.rs	91.30% <91.30%> (ø)
... and 64 more

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