Better support for Into

[BrynCooke]

Currently if a collection has elements that are E: Into<Something> then the first time that the field is set the builder will fix the type of E.

This could be improved by making the transient collection of type <Something> and performing the into conversion when the parameter is set.

The downside is that more fancy generic manipulation will need to be done.

[BrynCooke]

Another idea. All single entries to collections that are not a generic or numeric type is automatically given the Into treatment.
This fixes Strings and also gives the user the opportunity to define their own intermediate type.
For instance, if trying to construct a HeaderName you want to accept &str, String or HeaderName.
Define an enum:

enum IntoHeaderName {
  String(String),
  Str(&str),
  HeaderName(HeaderName)
}

Then From can be implemented for IntoHeaderName for HeaderName and dealt with in the constructor.