parser: parse let statements

- TODO: Type expressions
token: Add `mut` keyword

libconlang/src/token/token_type.rs

@@ -83,6 +83,7 @@ pub enum Keyword {
     If,
     In,
     Let,
+    Mut,
     Return,
     True,
     While,
@@ -167,6 +168,7 @@ impl Display for Keyword {
             Self::If => "if".fmt(f),
             Self::In => "in".fmt(f),
             Self::Let => "let".fmt(f),
+            Self::Mut => "mut".fmt(f),
             Self::Return => "return".fmt(f),
             Self::True => "true".fmt(f),
             Self::While => "while".fmt(f),
@@ -187,6 +189,7 @@ impl FromStr for Keyword {
             "if" => Self::If,
             "in" => Self::In,
             "let" => Self::Let,
+            "mut" => Self::Mut,
             "return" => Self::Return,
             "true" => Self::True,
             "while" => Self::While,