Introduce StaticType::Variadic for variadic parameters

This commit introduces `StaticType::Variadic` to represent variadic
parameter types in function signatures. This allows for more precise
type checking of functions that accept a variable number of arguments,
such as arithmetic operators.

Previously, variadic functions were handled using `StaticType::Any`,
which was too permissive and could lead to runtime errors. The new
`Variadic` type enforces that all arguments must conform to a specified
inner type.

Changes include:

- Adding `StaticType::Variadic` to `src/ast/types.rs`.
- Updating the type checker to handle `Variadic` types correctly when
  unifying with tuples or single arguments.
- Modifying built-in functions like arithmetic operators to use
  `Variadic` for their parameter types.
- Improving error messages for function call mismatches to be more
  specific.
- Adding a new test case to ensure arithmetic type mismatches are caught
  at compile time.
- A new example `data_stream_pipe_buffered.myc` is added.
- The `HMA.myc` example now has a `Skip: output` directive.

examples/HMA.myc

@@ -1,5 +1,6 @@
 ;; Benchmark: 98.9us
 ;; Benchmark-Repeat: 22
+;; Skip: output
 (do
   ;; make-sma Factory (O(1))
   (def make-sma

examples/data_stream_pipe_buffered.myc

@@ -0,0 +1,18 @@
+;; Skip: data output to stdout
+(do
+  (def EURUSD (create-m1-stream "EURUSD" (date "2020-01-03 09:30:00") (date "2020-01-03 10:00:00")))
+  (def GER40 (create-m1-stream "GER40" (date "2020-01-03 09:30:00") (date "2020-01-03 10:00:00")))
+
+  (def cnt 1)
+
+  (pipe-buffered 2 [EURUSD GER40]
+    (fn [eu ge]
+      (do
+        (def dax (.close (ge 0)))
+        (def eur (- dax (ge 1)))
+        (print cnt ": dax=" eur " (" (/ eur (.close (eu 0))) "$)" )
+        (assign cnt (+ cnt 1))
+      )
+    )
+  )
+)