Change "toolchain" to "python" (#4735)

Whew this is a lot.

The user-facing changes are:

- `uv toolchain` to `uv python` e.g. `uv python find`, `uv python
install`, ...
- `UV_TOOLCHAIN_DIR` to` UV_PYTHON_INSTALL_DIR`
- `<UV_STATE_DIR>/toolchains` to `<UV_STATE_DIR>/python` (with
[automatic
migration](https://github.com/astral-sh/uv/pull/4735/files#r1663029330))
- User-facing messages no longer refer to toolchains, instead using
"Python", "Python versions" or "Python installations"

The internal changes are:

- `uv-toolchain` crate to `uv-python`
- `Toolchain` no longer referenced in type names
- Dropped unused `SystemPython` type (previously replaced)
- Clarified the type names for "managed Python installations"
- (more little things)

crates/uv-python/src/pointer_size.rs

@@ -0,0 +1,21 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Copy, Clone, Serialize, Deserialize)]
+pub enum PointerSize {
+    /// 32-bit architecture.
+    #[serde(rename = "32")]
+    _32,
+    /// 64-bit architecture.
+    #[serde(rename = "64")]
+    _64,
+}
+
+impl PointerSize {
+    pub const fn is_32(self) -> bool {
+        matches!(self, Self::_32)
+    }
+
+    pub const fn is_64(self) -> bool {
+        matches!(self, Self::_64)
+    }
+}