Added more functions for introspecting pads of a component

[callendorph]

These tools are for constructing keepouts around components or parts of components, typically from a wrapping module.

[callendorph]

I need to more forward to meet deadlines. I'm going to merge. Please let me know if there is anything I need to address in a follow on PR before a release.

[tjknoth]

Only possible change is to match and throw an exception instead of cast -- slightly scary to expose a blind cast via public API

[callendorph]

OK - perhaps I'm being too cavalier with the as operator. Will that fatal if it doesn't match correct ?

[tjknoth]

I'm pretty sure it will fatal, and I think weird stuff can happen in optimized mode

Honestly this function seems a little specific for jsl; it feels more like user code. We could actually provide a more general version of this function in jsl instead.
There are two options:

downcast-objs<T> (objs:Seq<JITXObject>) -> Seq<T>, which matches all objects in the sequence against the explicit type parameter

downcast-objs<?T> (objs:Seq<JITXObject&?T> -> Seq<T> which infers the subtype of JITXObject -- but might not always infer the strongest type T if you give it a heterogeneous collection

[callendorph]

Ya - so I think this function should be in JITX not JSL if anywhere.
In this particular case, I'm confident that this JITXObject is a LandPatternPad. I could put this code in the design but it is going to be duplicated everywhere I use this pattern.
The other option is to change the bounds function to accept a JITXObject but that really just moves the problem. When bounds tries to inspect a LandPatternPad and it doesn't find one - then an error will be raised there. I think the jsl/landpatterns/leads/bounds function was probably written when I was trying for typing - but perhaps I need to re-evaluate that instead of adding this function.