[meta] Make more things pub(crate) instead of pub;

This could help the compiler find unused fields/methods. It didn't find any
during this migration.

cranelift/codegen/meta/src/gen_registers.rs

@@ -133,7 +133,7 @@ fn gen_isa(isa: &TargetIsa, fmt: &mut Formatter) {
     fmtln!(fmt, "}");
 }
 
-pub fn generate(isa: &TargetIsa, filename: &str, out_dir: &str) -> Result<(), error::Error> {
+pub(crate) fn generate(isa: &TargetIsa, filename: &str, out_dir: &str) -> Result<(), error::Error> {
     let mut fmt = Formatter::new();
     gen_isa(&isa, &mut fmt);
     fmt.update_file(filename, out_dir)?;