Add inferred constructors

[tuckermcclure]

Summary

Generated Julia messages can now infer length parameters from constructor inputs. For example, ControlParameters(motors = motors, ...) obtains N from the vector length, while a field declared as MotorParameters{N}[M] obtains N from its concrete elements and M from the collection length.

Design

• Inferred positional and keyword constructors delegate to the existing explicit Type{N,...} constructor. That constructor remains available and continues to perform length validation, field conversion, and storage reordering.
• When several inputs imply the same parameter, the constructor checks that they agree. Arrays of nested messages check every element.
• An inferred constructor is emitted only when every parameter can be determined from the inputs. Otherwise, callers use the explicit constructor.
• The array check uses a private loop binding that cannot collide with an IDL field or parameter name.

C++ generation and the generated type layouts are unchanged.

Validation

julia --project=test test/runtests.jl passes locally, including compiled C++ interoperability tests. Constructor tests cover regular Julia vectors, nested scalar messages, arrays and matrices of nested messages, conflicting lengths, keyword calls, and a parameter that cannot be inferred.

[tuckermcclure]

I'm going ahead with the merge because this is working great for me. Let me know if you see anything weird here though!