WIP: Re-implement parser in pest

Parser implementation is kind of iffy. Let's try to re-implement it
using pest.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/syn/peg.rs

@@ -0,0 +1,53 @@
+use crate::syn::ast::*;
+use crate::syn::token::*;
+use pest::{error::Error, iterators::Pair, Parser};
+
+#[derive(pest_derive::Parser)]
+#[grammar = "syn/parser.pest"]
+pub struct SybilParser;
+
+pub type Result<T, E = Error<Rule>> = std::result::Result<T, E>;
+
+fn parse_atom(pair: Pair<Rule>) -> Result<SpAtom> {
+    match pair.as_rule() {
+        Rule::float => todo!(),
+        Rule::int => todo!(),
+        Rule::assign => todo!(),
+        Rule::word => todo!(),
+        Rule::str => todo!(),
+        _ => unreachable!(),
+    }
+}
+
+fn parse_expr(pair: Pair<Rule>) -> Result<SpExpr> {
+    match pair.as_rule() {
+        Rule::atom => {
+            todo!()
+        }
+        Rule::quote => {
+            todo!()
+        }
+        Rule::apply => {
+            todo!()
+        }
+        _ => unreachable!(),
+    }
+}
+
+fn parse_stmt(pair: Pair<Rule>) -> Result<SpStmt> {
+    match pair.as_rule() {
+        Rule::expr => {
+            todo!()
+        }
+        _ => unreachable!(),
+    }
+}
+
+pub fn parse_file(text: &str) -> Result<Vec<SpStmt>> {
+    let input = SybilParser::parse(Rule::file, text)?.next().unwrap();
+    let mut stmts = Vec::new();
+    for pair in input.into_inner() {
+        stmts.push(parse_stmt(pair)?);
+    }
+    Ok(stmts)
+}