proposal(ir): state based implementation

docs/proposals/20221007-ir-state.md

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+# Refactor the IR to use state
+
+Author:
+- [kemingy](https://github.com/kemingy/)
+
+
+## Summary
+
+This proposal is designed to refactor the IR, as we discussed in [envd/#91](https://github.com/tensorchord/envd/issues/91#issuecomment-1239405359).
+
+## Motivation
+
+* Currently, we build the image with hard-coded order and parallelism. It's not flexible enough for some use cases.
+* The design of buildkit LLB state is suitable for method chaining implementation. Users can define the dependencies and parallelism easily.
+
+## Goals
+
+* Able to define the order and parallelism.
+* Provide low level APIs to build the image from scratch.
+
+## API
+
+```python
+def conda_env(root, version="3.10"):
+    conda = root.env("MAMBA_ROOT_PREFIX", "/opt/conda").run(
+        [
+            "curl micro.mamba.pm/install.sh | sh -s -- -y",
+            "/opt/conda/bin/micromamba create -n envd python={}".format(version),
+        ]
+    )
+    return conda
+
+
+def install_shell(root, sh="bash"):
+    if sh == "bash":
+        return root
+
+
+def install_vscode_ext(ext=()):
+    root = Source.merge(Source.http("openvsx/{}".format(x), filename=x) for x in ext)
+    return root
+
+
+def install_key():
+    root = (
+        Source.scratch()
+        .mkdir("/var/envd", permission=755)
+        .file("/var/envd/authorized_keys", data="xxx envd", permission=644)
+    )
+    return root
+
+
+def parallel_build(root):
+    """demo for parallel + diff + merge"""
+    # use `root.state()` to create a copy explicitly
+    conda = conda_env(install_shell(root.state()))
+    vscode = install_vscode_ext()
+    key = config_key()
+    return Source.merge([root, key, vscode, Diff(root, conda)])
+
+
+def build():
+    root = (
+        Source.image("ubuntu:22.04")
+        .apt_packages(
+            ["curl", "wget"], update=True, clean=True, without_recommends=True
+        )
+        .run(["curl https://starship.rs/install.sh | sh -s -- -y"], mount=None)
+        .copy(
+            Source.image("tensorchord/envd-sshd-from-scratch:latest"),
+            source_path="/usr/bin/envd-sshd",
+            dest_path="/var/envd/bin/envd-sshd",
+        )
+    )
+    return parallel_build(root)
+```
+
+## Risk
+
+* Implementation in Starlark
+* VSCode language server