feat(api): Introduce `fold`/`map` functions for AST traversal

[byakuren-hijiri]

This improves compiler's API providing an interface to traverse the AST. It should be a part of the compiler, since it is be useful for devtools using the API and might be useful internally.

See: #314

• [x] I have updated CHANGELOG.md
• [ ] I have added tests to demonstrate the contribution is correctly implemented: this usually includes both positive and negative tests, showing the happy path(s) and featuring intentionally broken cases
• [x] I have run all the tests locally and no test failure was reported
• [x] I did not do unrelated and/or undiscussed refactorings

[byakuren-hijiri]

Here are some examples of usage of these functions:

• https://github.com/nowarp/misti/blob/66f4254b561d1d4f3058974c06f7f4b5a4215f09/src/detectors/builtin/zeroAddress.ts#L38
• https://github.com/nowarp/misti/blob/66f4254b561d1d4f3058974c06f7f4b5a4215f09/src/detectors/builtin/readOnlyVariables.ts#L117

They could be used to traverse the AST by calling callback functions or accumulating information about the AST, so they are quite useful for all the tools working with Tact AST. Implementing this API in the compiler will make it easier to maintain the API upon AST changes.