Add design proposal for basic interfaces. (#2229)

This change adds a `docs/proposals/` directory to give us a place to iterate on proposals for language/compiler features that are a bit too large to handle as issues.
The first proposal I'm checking in here is a sketch of what our built-in numeric interfaces might want to look like.

I do not propose that the design in the document is perfect or even *finished*.
The goal here is to get things checked into source control to foster subsequent discussion and iteration.

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+Proposals
+=========
+
+This directory contains proposals / "RFCs" for Slang language/compiler/system features.
+In general, proposals are used for features that are large or complicated enough that the design and/or plan benefits from being discussed in detail before we commit to making code changes.
+
+Design *discussion* can often be facilitated by a GitHub/GitLab issue, or PR review comments, but often it is difficult for a developer to get a clear summary of the final decisions/POR without reading an entire discussion thread.
+By framing design discussion around a document that captures the decisions, we hope to ensure that the at the end of the discussion we have useful collateral for anybody who goes to implement a feature.