Merge pull request/issue templates

.github/ISSUE_TEMPLATE/blank-issue.md

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+---
+name: Blank Issue
+about: Create a blank issue.
+---

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

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+---
+name: "Cranelift Bug report"
+about: "Report a bug or a crash in Cranelift."
+labels: 'bug'
+---
+
+Thanks for opening a bug report! 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...)

.github/ISSUE_TEMPLATE/improvement.md

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+---
+name: "Improvement"
+about: "A feature request or code improvement."
+---
+
+<!-- Please try to describe precisely what you would like to do in
+Cranelift/Wasmtime 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! -->
+
+#### Feature
+
+<!-- What is the feature or code improvement you would like to do in
+Cranelift/Wasmtime? -->
+
+#### Benefit
+
+<!-- What is the value of adding this in Cranelift/Wasmtime? -->
+
+#### Implementation
+
+<!-- Do you have an implementation plan, and/or ideas for data structures or
+algorithms to use? -->
+
+#### Alternatives
+
+<!-- Have you considered alternative implementations? If so, how are they
+better or worse than your proposal? -->