[contribution] Add Github issue templates;

cranelift/.github/ISSUE_TEMPLATE/bug-report.md

@@ -0,0 +1,16 @@
+---
+name: "Bug report"
+about: "Report a bug or a crash in Cranelift."
+labels: 'bug'
+---
+
+Thanks for opening a bug report on Cranelift! Please answer the questions below
+if they're relevant and delete this text before submitting.
+
+- What are the steps to reproduce the issue? Can you include a CLIF test case,
+  ideally reduced with the `bugpoint` clif-util command?
+- What do you expect to happen? What does actually happen? Does it panic, and
+  if so, with which assertion?
+- Which Cranelift version / commit hash / branch are you using?
+- If relevant, can you include some extra information about your environment?
+  (Rust version, operating system, architecture...)

cranelift/.github/ISSUE_TEMPLATE/improvement.md

@@ -0,0 +1,15 @@
+---
+name: "Improvement"
+about: "A feature request or code improvement."
+---
+
+Please try to describe precisely what you would like to do in Cranelift and/or
+expect from it. You can answer the questions below if they're relevant and
+delete this text before submitting. Thanks for opening an issue!
+
+- What is the feature or code improvement you would like to do in Cranelift?
+- What is the value of adding this in Cranelift?
+- Do you have an implementation plan, and/or ideas for data structures or
+  algorithms to use?
+- Have you considered alternative implementations? If so, how are they better
+  or worse than your proposal?