Increase MSRV to 1.64.0 and pin generated bindings rust version

**Description**
 - Increase MSRV to `1.64.0` and add README line stating MSRV.
 - Using the `bindgen::Builder::rust_target` function, set the rustc version
   required to `1.64.0` to access FFI types from `core`.

**Motivation**
Previous MSRV bump to `1.57.0` failed because `bindgen` generated bindings
which required `1.64.0`. See issue
rust-lang/rust-bindgen#2324 for slightly more
context.

**Testing Done**
 - `cargo +1.57.0 build` fails
 - `cargo +1.64.0 build` succeeds

Author: Declan Kelly

.github/workflows/rust-ci.yml

@@ -17,7 +17,7 @@ jobs:
           - stable
           - beta
           - nightly
-          - 1.57.0
+          - 1.64.0
         os:
           - ubuntu-latest
           - windows-latest

README.md

@@ -57,6 +57,8 @@ See [`examples/`](https://github.com/declanvk/reckoner/tree/master/examples) for
 
 ## Crates
 
+The MSRV for both crates is `1.64.0`.
+
 ### `reckoner`
 
 [![crates.io](https://img.shields.io/crates/d/reckoner)](https://crates.io/crates/reckoner) [![docs.rs](https://docs.rs/reckoner/badge.svg)](https://docs.rs/reckoner)

creachadair-imath-sys/build.rs

@@ -42,6 +42,8 @@ fn main() {
         .allowlist_function(FUNCTION_REG)
         .allowlist_type(TYPE_REG)
         .allowlist_var(VAR_REG)
+        // Current MSRV is 1.64.0, see top-level README.md file
+        .rust_target(bindgen::RustTarget::Stable_1_64)
         .generate()
         .expect("Unable to generate bindings");