feat: refactor some methods and modules

* move uplc::ast::builder to uplc::builder
* rename aiken_lang::uplc to aiken_lang::gen_uplc
* move aiken_lang::air and aiken_lang::builder to aiken_lang::gen_uplc
  as submodules

Co-authored-by: Kasey White <kwhitemsg@gmail.com>

crates/aiken-lang/src/gen_uplc/stack.rs

@@ -0,0 +1,68 @@
+use crate::IdGenerator;
+
+use super::air::Air;
+
+pub struct AirStack<'a> {
+    pub id_gen: &'a mut IdGenerator,
+    pub scope: Vec<u64>,
+    pub air: Vec<Air>,
+}
+
+impl<'a> AirStack<'a> {
+    pub fn new(id_gen: &'a mut IdGenerator) -> Self {
+        AirStack {
+            id_gen,
+            scope: vec![0],
+            air: vec![],
+        }
+    }
+
+    pub fn with_scope(id_gen: &'a mut IdGenerator, scope: Vec<u64>) -> Self {
+        AirStack {
+            id_gen,
+            scope,
+            air: vec![],
+        }
+    }
+
+    pub fn merge(mut self, other: AirStack) -> Self {
+        self.air.extend(other.air.into_iter());
+
+        self
+    }
+
+    pub fn int(self, value: String) -> Self {
+        let mut air = self.air.clone();
+
+        air.push(Air::Int {
+            scope: self.scope.clone(),
+            value,
+        });
+
+        AirStack {
+            id_gen: self.id_gen,
+            scope: self.scope,
+            air,
+        }
+    }
+
+    pub fn string(mut self, value: String) -> Self {
+        self.air.push(Air::String {
+            scope: self.scope.clone(),
+            value,
+        });
+
+        self
+    }
+
+    pub fn byte_array(mut self, bytes: Vec<u8>) -> Self {
+        self.air.push(Air::ByteArray {
+            scope: self.scope.clone(),
+            bytes,
+        });
+
+        self
+    }
+
+    // pub fn sequence(mut self, expressions: AirEnv) -> Self {}
+}