Add the start of a hindley-milner typechecker

The beginning of a parse-don't-validate-based hindley-milner
typechecker, which returns on success an IR where every AST node
trivially knows its own type, and using those types to determine LLVM
types in codegen.

src/compiler.rs

@@ -8,7 +8,7 @@ use test_strategy::Arbitrary;
 
 use crate::codegen::{self, Codegen};
 use crate::common::Result;
-use crate::parser;
+use crate::{parser, tc};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Arbitrary)]
 pub enum OutputFormat {
@@ -55,6 +55,8 @@ pub struct CompilerOptions {
 pub fn compile_file(input: &Path, output: &Path, options: &CompilerOptions) -> Result<()> {
     let src = fs::read_to_string(input)?;
     let (_, decls) = parser::toplevel(&src)?; // TODO: statements
+    let decls = tc::typecheck_toplevel(decls)?;
+
     let context = codegen::Context::create();
     let mut codegen = Codegen::new(
         &context,