Leon template library (#766)

* leon: first implementation

* Update crates/leon/src/values.rs

Co-authored-by: Jiahao XU <Jiahao_XU@outlook.com>

* Workaround orphan rules to make API more intuitive

* Fmt

* Clippy

* Use CoW

* Use cow for items too

* Test that const construction works

* leon: Initial attempt at O(n) parser

* leon: finish parser (except escapes)

* leon: Improve ergonomics of compile-time templates

* Document helpers

* leon: Docs tweaks

* leon: Use macro to minimise parser tests

* leon: add escapes to parser

* leon: test escapes preceding keys

* leon: add multibyte tests

* leon: test escapes following keys

* Format

* Debug

* leon: Don't actually need to keep track of the key

* leon: Parse to vec first

* leon: there's actually no need for string cows

* leon: reorganise and redo macro now that there's no coww

* Well that was silly

* leon: Adjust text end when pushing

* leon: catch unbalanced keys

* Add error tests

* leon: Catch unfinished escape

* Comment out debugging

* leon: fuzz

* Clippy

* leon: Box parse error

* leon: &dyn instead of impl

* Can't impl FromStr, so rename to parse

* Add Vec<> to values

* leon: Add benches for ways to supply values

* leon: Add bench comparing to std and tt

* Fix fuzz

* Fmt

* Split ParseError and RenderError

* Make miette optional

* Remove RenderError lifetime

* Simplify ParseError type schema

* Write concrete Values types instead of generics

* Add license files

* Reduce criterion deps

* Make default a cow

* Add a CLI leon tool

* Fix tests

* Clippy

* Disable cli by default

* Avoid failing the build when cli is off

* Add to ci

* Update crates/leon/src/main.rs

Co-authored-by: Jiahao XU <Jiahao_XU@outlook.com>

* Update crates/leon/Cargo.toml

Co-authored-by: Jiahao XU <Jiahao_XU@outlook.com>

* Bump version

* Error not transparent

* Diagnostic can do forwarding

* Simplify error type

* Expand doc examples

* Generic Values for Hash and BTree maps

* One more borrowed

* Forward implementations

* More generics

* Add has_keys

* Lock stdout in leon tool

* No more debug comments in parser

* Even more generics

* Macros to reduce bench duplication

* Further simplify error

* Fix leon main

* Stable support

* Clippy

---------

Co-authored-by: Jiahao XU <Jiahao_XU@outlook.com>

crates/leon/src/macros.rs

@@ -0,0 +1,50 @@
+/// Construct a template constant without needing to make an items constant.
+///
+/// This is essentially a shorthand for:
+///
+/// ```
+/// use leon::{Item, Template};
+/// Template::new({
+///     const ITEMS: &'static [Item<'static>] = &[Item::Text("Hello "), Item::Key("name")];
+///     ITEMS
+/// }, Some("world"));
+/// ```
+///
+/// # Examples
+///
+/// ```
+/// use leon::Item::*;
+/// assert_eq!(
+///     leon::template!(Text("Hello "), Key("name"))
+///         .render(&[("name", "Магда Нахман")])
+///         .unwrap(),
+///     "Hello Магда Нахман",
+/// );
+/// ```
+///
+/// With a default:
+///
+/// ```
+/// use leon::Item::*;
+/// assert_eq!(
+///     leon::template!(Text("Hello "), Key("name"); "M. P. T. Acharya")
+///         .render(&[("city", "Madras")])
+///         .unwrap(),
+///     "Hello M. P. T. Acharya",
+/// );
+/// ```
+#[macro_export]
+macro_rules! template {
+    ($($item:expr),* $(,)?) => {
+        $crate::Template::new({
+            const ITEMS: &'static [$crate::Item<'static>] = &[$($item),*];
+            ITEMS
+        }, ::core::option::Option::None)
+    };
+    ($($item:expr),* $(,)? ; $default:expr) => {
+        $crate::Template::new({
+            const ITEMS: &'static [$crate::Item<'static>] = &[$($item),*];
+            ITEMS
+        }, ::core::option::Option::Some($default))
+    };
+}