Rename dir: compiler -> crates

crates/erg_linter/README.md

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+# erg-linter (WIP)
+
+erg-linter (can be used with `erg lint`) is a tool to check the erg file for errors.
+
+## Features
+
+The following codes are warned.
+
+* Unreachable codes
+* Wildcard import
+* Unused variables
+* Shadowing of built-in variables
+* Unused objects that are not `NoneLike`
+* Procedures without side-effects
+* Variables that can be defined as constants
+* Unnecessary `.clone`
+* Mutable objects that do not change
+* Hardcoded well-known constants (e.g. `3.14`)
+* Defining a subroutine with too many parameters

crates/erg_linter/lint.rs

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+use erg_common::Str;
+
+use crate::error::CompileWarnings;
+use crate::hir::HIR;
+
+#[derive(Debug, Default)]
+pub struct Linter {
+    _used: Vec<Str>,
+}
+
+impl Linter {
+    pub fn new() -> Self {
+        Self { _used: Vec::new() }
+    }
+
+    pub fn lint(&mut self, _hir: &HIR) -> CompileWarnings {
+        CompileWarnings::empty()
+    }
+}

crates/erg_linter/mod.rs

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+mod lint;
+
+pub use lint::Linter;