Add fluxc compiler skeleton: token definitions and lexer

Introduces the fluxc Rust crate with the first two compiler stages:

- token.rs: define_tokens! macro generates TokenKind enum and its
  Display impl from a single table covering all Flux tokens
  (literals, keywords, operators, punctuation, Eof/Unknown).
  Span (half-open u32 byte range) and Token<'src> (kind + span +
  zero-copy text slice) round out the module.

- lexer.rs: Lexer<'src> produces Token<'src> from a source &str.
  Skips whitespace, // line comments, and /* */ block comments.
  Handles all integer bases (decimal, hex, octal, binary with _
  separators), floats (fractional + exponent), string/char literals
  with escape sequences, and Unicode identifiers via unicode-xid.
  Implements Iterator<Item = Token> and includes 17 unit tests.

Also adds .gitignore (ignores fluxc/target) and expands
examples/fibonacci.flx with an iterative variant.

examples/fibonacci.flx

@@ -1,7 +1,23 @@
-fn fibonacci(n: u8) -> u64 {
+fn fibonacci_rec(n: u8) -> u64 {
     if n < 2 {
         return n;
     }
 
-    return fibonacci(n - 1) + fibonacci(n - 2);
+    return fibonacci_rec(n - 1) + fibonacci_rec(n - 2);
+}
+
+fn fibonacci_iter(n: u8) -> u64 {
+    let mut counter = 0;
+    let mut a = 0;
+    let mut b = 1;
+
+    while counter < n {
+        let temp = a + b;
+        a = b;
+        b = temp;
+
+        counter = counter + 1;
+    }
+
+    return a;
 }