Clean up fish-printf in preparation for publishing

Make fish-printf no longer depend on the widestring crate, as other clients
won't use it; instead this is an optional feature.

Make format strings a generic type, so that both narrow and wide strings can
serve. This removes a lot of the complexity around converting from narrow to
wide.

Add a README.md to this crate.

printf/README.md

@@ -0,0 +1,43 @@
+# fish-printf
+
+The printf implementation used in [fish-shell](https://fishshell.com), based on musl printf.
+
+[![crates.io](https://img.shields.io/crates/v/fish-printf.svg)](https://crates.io/crates/fish-printf)
+
+Licensed under the MIT license.
+
+### Usage
+
+Run `cargo add fish-printf` to add this crate to your `Cargo.toml` file.
+
+Also run `cargo add widestring` to add the widestring crate.
+
+### Notes
+
+fish-printf attempts to match the C standard for printf. It supports the following features:
+
+-   Locale-specific formatting (decimal point, thousands separator, etc.)
+-   Honors the current rounding mode.
+-   Supports the `%n` modifier for counting characters written.
+
+fish-printf does not support positional arguments, such as `printf("%2$d", 1, 2)`.
+
+Prefixes like `l` or `ll` are recognized, but only used for validating the format string.
+The size of integer values is taken from the argument type.
+
+fish-printf can output to an `std::fmt::Write` object, or return a string.
+
+For reasons related to fish-shell, fish-printf has a feature "widestring" which uses the [widestring](https://crates.io/crates/widestring) crate. This is off by default.
+
+### Examples
+
+```rust
+use fish_printf::sprintf;
+
+// Create a `String` from a format string.
+let s = sprintf!("%0.5g", 123456.0) // 1.2346e+05
+
+// Append to an existing string.
+let s = String::new();
+sprintf!(=> &mut s, "%0.5g", 123456.0) // 1.2346e+05
+```