Refactoring

[plar]

In this PR, I've implemented several long-desired refactorings:

Build and Continuous Integration

• Replaced the Makefile with a bash make script. The previous targets were all PHONY, and the Makefile syntax wasn't ideal.
• Added GitHub Actions for continuous integration and automation.

Go Environment and Tools

• Integrated Go ecosystem tools for quality assurance, including mod, fmt, vet, lint, staticcheck, vulncheck, and gogec.
• Updated go.mod to Go version 1.18.
• Updated Go dependencies.

Code Refactoring

• Simplified node structures and merged the zeroChild field into the children field.
• Simplified code by eliminating bit operations for certain nodes.
• Merged similar functionality between the ForEach method and the Iterator.
• Split node.go and tree.go into separate files for improved modularity.
• Refactored and simplified tree iteration and traversal functionality.
• Refactored and simplified the String method for the tree.

Testing and Quality Assurance

• Introduced golden files for tests.
• Added t.Parallel for running tests in parallel.
• Refactored some tests for better coverage and readability.

Code Quality

• Fixed numerous Go lint warnings.
• Added comments to internal code for clarity.
• Renamed variables and method names to improve readability. (After all, cache invalidation and naming things are the two hard things in computer science. :)

This refactoring effort enhances code structure and quality without altering the external interface/public API.