This commit introduces a new test suite for HM Let-Polymorphism.
These tests verify that user-defined generic functions work correctly
across multiple series element types without type conflicts. This is
crucial for ensuring that the type inference system can handle
polymorphic functions properly, especially when they are applied to
different types. The tests cover scenarios like generic accessors,
runtime correctness with different series types, record series, and
higher-order functions. They also include a test for the value
restriction to ensure that series bindings are not generalized, which
would lead to incorrect type errors.
