StackStack and CI Updates

This commit adds a new data type to the codebase. It organizes data
into first in last out linked lists of first in last out linked lists.
Hence the name: StackStack.

This data type was made to represent frames of execution, where there
there is a stack for each function call containing local variables and
arguments. The following tertiary goals were also met:
- no relocating of adjacent data on push or pop
- no copying or cloning of contained data on modification or mutation
- index access to all elements of all contained stacks starting with
  most recent insertions.

There are operations to allocate a new stack on the stackstack and
to deallocate the top stack on the stackstack. Additionally there are
operations for pushing and popping from the top stack.

Unit tests are added and CI is updated to include them.

Signed-off-by: Ava Affine <ava@sunnypup.io>

.gitlab-ci.yml

@@ -3,7 +3,8 @@ default:
 
 stages:
   - build
-  - test
+  - test-frontend
+  - test-backend
 
 compile-library:
   stage: build
@@ -15,14 +16,22 @@ compile-decomposer:
   script:
     - cargo build --bin decomposer
 
-unit-test-language-frontend:
-  stage: test
+unit-test-lexer:
+  stage: test-frontend
   script:
     - cargo test lexer
+
+unit-test-parser:
+  stage: test-frontend
+  script:
     - cargo test parser
 
 unit-test-number-package:
-  stage: test
+  stage: test-backend
   script:
     - cargo test number
 
+unit-test-stackstack:
+  stage: test-backend
+  script:
+    - cargo test stackstack