Refactor conversion templates into classes

[HertzDevil]

This patch, split from #78, promotes conversion templates (Bindgen::Call::Result#conversion) to their own class hierarchy. This allows templates to perform complex substitutions that Util.template cannot achieve.

The main issue with Util.template is that it only supports simple string substitutions (e.g. %.to_unsafe, Converter.unwrap(%)). Passing a wrapper-Proc to a CrystalProc is an example that cannot be expressed in this way:

• The binding expects a binding-Proc, which takes binding types and returns a binding type;
• The user-provided wrapper-Proc takes and returns wrapper types.
• Thus, the binding-Proc needs to convert each argument to its wrapper type, invoke _proc_, then convert the return value to its binding type;
• The binding-Proc itself needs to be converted into its binding type (CrystalProc).

Since the conversion must occur every time a wrapper-Proc is passed to the binding, the best place to perform the conversion is through the template facility, rather than in a Call::Body. However, as noted in the original bug report, such Proc conversions are already properly handled in the jumptables (Processor::VirtualOverride::JumptableHook), just not in the Qt processor. Therefore, the template for wrapper-Procs actually ends up using CallBuilder::CrystalFromCpp directly; it works because that Call::Body generates an expression, rather than a complete code block.

Apart from Proc, I expect that some other generic types will also require extra conversions beyond Util.template's capability. This is why I suggested that Slice may also use this feature as well.

Template::None and Template::Sequence are utility templates that either perform no substitutions at all, or chain multiple templates together. They make the composition of templates more concise. (Now that I think about it, Bindgen::Call::Result#conversion itself could be an Array(Template::Base) too, although #followed_by will be replaced with array concatenation.)

[Papierkorb]

Please have a look at the comments. Overall this is much much more manageable for me, thanks for splitting it up!

[HertzDevil]

Please take a look if anything is still missing.

I rebased the original CrystalProc fix onto my branch, ready for a new PR, by the way: https://github.com/HertzDevil/bindgen/tree/fix-crystalproc-template

[Papierkorb]

Oops, thanks for the ping