Improving icu_harfbuzz compiled data (#4574)

The current HB compiled data code is not ideal, because it uses
`static_to_owned`, which might be blocking optimisations. This also let
me realise that some compiled constructors that could be `const` aren't.

CHANGELOG.md

@@ -11,7 +11,7 @@
     - `FormattedDateTime` and `FormattedZonedDateTime` now implement `Clone` and `Copy` (https://github.com/unicode-org/icu4x/pull/4476)
   - `icu_properties`
     - Add `Aran` script code (https://github.com/unicode-org/icu4x/pull/4426)
-    - Mark `BidiClassAdapter::new` as const (https://github.com/unicode-org/icu4x/pull/4584)
+    - Mark additional constructors as `const` (https://github.com/unicode-org/icu4x/pull/4584, https://github.com/unicode-org/icu4x/pull/4574)
     - Implement Joining_Type property (https://github.com/unicode-org/icu4x/pull/4599)
   - `icu_segmenter`
     - Fix Unicode 15.0 line breaking (https://github.com/unicode-org/icu4x/pull/4389)

components/normalizer/src/properties.rs

@@ -90,7 +90,7 @@ impl CanonicalComposition {
     ///
     /// [📚 Help choosing a constructor](icu_provider::constructors)
     #[cfg(feature = "compiled_data")]
-    pub fn new() -> Self {
+    pub const fn new() -> Self {
         Self {
             canonical_compositions: DataPayload::from_static_ref(
                 crate::provider::Baked::SINGLETON_NORMALIZER_COMP_V1,

components/properties/src/props.rs

@@ -585,7 +585,7 @@ macro_rules! impl_value_getter {
         impl $ty {
             $(#[$attr_n2e])*
             #[cfg(feature = "compiled_data")]
-            $vis_n2e fn $cname_n2e() -> PropertyValueNameToEnumMapperBorrowed<'static, $ty> {
+            $vis_n2e const fn $cname_n2e() -> PropertyValueNameToEnumMapperBorrowed<'static, $ty> {
                 PropertyValueNameToEnumMapperBorrowed {
                     map: crate::provider::Baked::$singleton_n2e,
                     markers: PhantomData,
@@ -604,7 +604,7 @@ macro_rules! impl_value_getter {
             $(
                 $(#[$attr_e2sn])*
                 #[cfg(feature = "compiled_data")]
-                $vis_e2sn fn $cname_e2sn() -> $mapper_e2snb<'static, $ty> {
+                $vis_e2sn const fn $cname_e2sn() -> $mapper_e2snb<'static, $ty> {
                     $mapper_e2snb {
                         map: crate::provider::Baked::$singleton_e2sn,
                         markers: PhantomData,
@@ -622,7 +622,7 @@ macro_rules! impl_value_getter {
 
                 $(#[$attr_e2ln])*
                 #[cfg(feature = "compiled_data")]
-                $vis_e2ln fn $cname_e2ln() -> $mapper_e2lnb<'static, $ty> {
+                $vis_e2ln const fn $cname_e2ln() -> $mapper_e2lnb<'static, $ty> {
                     $mapper_e2lnb {
                         map: crate::provider::Baked::$singleton_e2ln,
                         markers: PhantomData,