uv-pep440: DRY up VersionSmall implementation

[BurntSushi]

This PR simplifies the VersionSmall implementation a bit by utilizing
more constants. That is, if the bit-level format changes, most of
those changes should be implementable by just changing the constants.
Previously, you would need to audit and tweak the code as well. (The
exception here is push_release. If the release segment bit format is
changed, then that function will need to be tweaked. I didn't think it
was worth over-complicating things to make its implementation more
general.)

[BurntSushi]

With this PR, to add a "local" suffix kind, I believe this is all that's needed (at least, for making changes to the bit-level representation):

$ git diff
diff --git a/crates/uv-pep440/src/version.rs b/crates/uv-pep440/src/version.rs
index 22b9730ed..6e51df393 100644
--- a/crates/uv-pep440/src/version.rs
+++ b/crates/uv-pep440/src/version.rs
@@ -922,22 +922,27 @@ impl VersionSmall {
     const SUFFIX_PRE_BETA: u64 = 3;
     const SUFFIX_PRE_RC: u64 = 4;
     const SUFFIX_NONE: u64 = 5;
-    const SUFFIX_POST: u64 = 6;
-    const SUFFIX_MAX: u64 = 7;
+    const SUFFIX_LOCAL: u64 = 6;
+    const SUFFIX_POST: u64 = 7;
+    const SUFFIX_MAX: u64 = 8;

     // The mask to get only the release segment bits.
     const SUFFIX_RELEASE_MASK: u64 = 0xFFFF_FFFF_FF00_0000;
     // The mask to get the version suffix.
-    const SUFFIX_VERSION_MASK: u64 = 0x001F_FFFF;
+    const SUFFIX_VERSION_MASK: u64 = 0x000F_FFFF;
     // The number of bits used by the version suffix. Shifting the `repr`
     // right by this number of bits should put the suffix kind in the least
     // significant bits.
-    const SUFFIX_VERSION_BIT_LEN: u64 = 21;
+    const SUFFIX_VERSION_BIT_LEN: u64 = 20;
     // The mask to get only the suffix kind, after shifting right by the
     // version bits. If you need to add a bit here, then you'll probably need
     // to take a bit from the suffix version. (Which requires a change to both
     // the mask and the bit length above.)
-    const SUFFIX_KIND_MASK: u64 = 0b111;
+    //
+    // NOTE: If you do change the bit format here, you'll need to bump any
+    // cache versions in uv that use rkyv with `Version` in them. That includes
+    // *at least* the "simple" cache.
+    const SUFFIX_KIND_MASK: u64 = 0b1111;

     #[inline]
     fn new() -> Self {

[charliermarsh]

Thank you! 🙏

[charliermarsh]

Thanks so helpful.