Implement all Expr unmarshalling (untested). Put wordsize logic in constants.ts. Some README additions.

src/runtime/c/README.md

@@ -2,7 +2,9 @@
 
 ## Requirements
 
-Required system packages (Debian/Ubuntu):
+### Debian/Ubuntu
+
+Required system packages (`apt install ...`):
 ```
 autoconf
 automake
@@ -11,6 +13,11 @@ make
 g++
 ```
 
+### macOS
+
+- Install XCode from App Store
+- Install XCode command line tools: `xcode-select --install`
+
 ## Installation
 
 Installing the runtime (puts libraries in `/usr/local/lib`):