feat: Partial explicit type instantiation

[Kamirus]

Add partial explicit type instantiation using wildcard for types that must be inferred.
Example:

VarArray.map<_, Nat>(testArray, func x = x * 2)

1. Specifies the 2nd type argument in order to avoid the invariance limitation (Otherwise could be ambiguous: Nat vs Int)
2. Omit the 1st type argument, let the compiler infer it

Partial type instantiations are an improvement over explicit return type annotations:

• Works well with the dot-notation (return type annotations needed extra parenthesis or intermediate let-binding)
• Shorter, more concise syntax

[github-actions]

Comparing from aa67d28 to f8ea484:
In terms of gas, no changes are observed in 5 tests.
In terms of size, no changes are observed in 5 tests.