Update examples for Zig

[kind84]

This PR updates the code examples for the Zig programming language.

Those examples where introduced in #242 but, since the Zig language is evolving at fast pace, they needed some adjustments:

• Replaced var with const for non-mutated variables
• Better implementation for Dijkstra's algorithm
• Removed async/await syntax as it is (temporarily) not provided any more by the language. Replaced by threads
• Updated standard library names and signatures
• Other small improvements

The code has been tested using zig version 0.14.0-dev.577+3344ed8b8.