feat: init state, io and std handler

Author: soywod

.github/workflows/tests.yml

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+name: Tests
+
+on:
+  push:
+
+jobs:
+  tests:
+    uses: pimalaya/nix/.github/workflows/tests.yml@master
+    secrets: inherit

.gitignore

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+# Cargo config directory
+.cargo/
+
+# Cargo build directory
+target/
+debug/
+
+# These are backup files generated by rustfmt
+**/*.rs.bk
+
+# MSVC Windows builds of rustc generate these, which store debugging information
+*.pdb
+
+# Nix build directory
+result
+result-*
+
+# Direnv
+/.envrc
+/.direnv
+
+# Covers JetBrains IDEs: IntelliJ, RubyMine, PhpStorm, AppCode, PyCharm, CLion, Android Studio, WebStorm and Rider
+# Reference: https://intellij-support.jetbrains.com/hc/en-us/articles/206544839
+.idea/
+
+# IntelliJ
+out/
+
+# mpeltonen/sbt-idea plugin
+.idea_modules/
+
+# JIRA plugin
+atlassian-ide-plugin.xml
+
+# Cursive Clojure plugin
+.idea/replstate.xml
+
+# SonarLint plugin
+.idea/sonarlint/
+
+# Crashlytics plugin (for Android Studio and IntelliJ)
+com_crashlytics_export_strings.xml
+crashlytics.properties
+crashlytics-build.properties
+fabric.properties
+
+## Others
+.metadata/

CONTRIBUTING.md

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+# Contributing guide
+
+Thank you for investing your time in contributing to the stream flows project.
+
+## Development
+
+The development environment is managed by [Nix](https://nixos.org/download.html).
+Running `nix-shell` will spawn a shell with everything you need to get started with the project.
+
+If you do not want to use Nix, you can either use [rustup](https://rust-lang.github.io/rustup/index.html):
+
+```text
+rustup update
+```
+
+or install manually the following dependencies:
+
+- [cargo](https://doc.rust-lang.org/cargo/)
+- [rustc](https://doc.rust-lang.org/stable/rustc/platform-support.html)
+
+## Build
+
+```
+cargo build
+```
+
+You can disable [default features](https://doc.rust-lang.org/cargo/reference/features.html) with `--no-default-features` and enable features with `--features feat1,feat2,feat3`.
+
+Finally, you can build a release with `--release`:
+
+```
+cargo build --no-default-features --release
+```
+
+## Commit style
+
+Stream flows follow the [conventional commits specification](https://www.conventionalcommits.org/en/v1.0.0/#summary).