First prototype for new JsString using UTF-16 (#1659)

I think it's time to address the elephant in the room.

This Pull Request will (hopefully!) solve part of #736.

This is a complete rewrite of `JsString`, but instead of storing `u8` bytes it stores `u16` words. The `encode!` macro (renamed to `utf16!` for simplicity) from the `const-utf16` crate allows us to create UTF-16 encoded arrays at compilation time. `JsString` implements `Deref<Target=[u16]>` to unlock the slice methods and possibly make some manipulations easier. However, we would need to create our own library of utilities for `JsString`.

Cargo.toml

@@ -9,6 +9,7 @@ members = [
     "boa_unicode",
     "boa_wasm",
     "boa_examples",
+    "boa_macros",
 ]
 
 [workspace.package]
@@ -26,6 +27,7 @@ boa_interner = { version = "0.16.0", path = "boa_interner" }
 boa_gc = { version = "0.16.0", path = "boa_gc" }
 boa_profiler = { version = "0.16.0", path = "boa_profiler" }
 boa_unicode = { version = "0.16.0", path = "boa_unicode" }
+boa_macros = { version = "0.16.0", path = "boa_macros" }
 
 [workspace.metadata.workspaces]
 allow_branch = "main"

boa_engine/Cargo.toml

@@ -32,6 +32,7 @@ boa_unicode.workspace = true
 boa_interner.workspace = true
 boa_gc.workspace = true
 boa_profiler.workspace = true
+boa_macros.workspace = true
 gc = "0.4.1"
 serde = { version = "1.0.145", features = ["derive", "rc"] }
 serde_json = "1.0.85"
@@ -50,6 +51,8 @@ unicode-normalization = "0.1.22"
 dyn-clone = "1.0.9"
 once_cell = "1.15.0"
 tap = "1.0.1"
+sptr = "0.3.2"
+static_assertions = "1.1.0"
 icu_locale_canonicalizer = { version = "0.6.0", features = ["serde"], optional = true }
 icu_locid = { version = "0.6.0", features = ["serde"], optional = true }
 icu_datetime = { version = "0.6.0", features = ["serde"], optional = true }