remove guard is let

proposals/0007-remove-guard-is-let.mbt.md

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+# Remove `guard`, `is`, `let` keyword
+
+
+## Introduction
+
+In the Moonbit language, the functionality of the `let` and `is` keyword overlaps. We can change `=` to match operator. With this, the `guard` keyword becomes unnecessary.
+
+To avoid confusion between assignment and matching, the `:` is required whether the type is specified or not. We use good old `<~` for assignment.
+
+
+## match operator
+
+```moonbit
+let a = 1234
+let b: Int = 1234
+guard x is Some(v)
+while m is Some(v) {}
+guard index >= 0 && index < array.length() else { None }
+```
+
+changes to
+
+```moonbit
+a := 1234
+b : Int = 1234
+Some(v) := x
+while Some(v) := m {}
+if not(index >= 0 && index < array.length()) { return None }
+```
+
+## assignment operator
+
+```moonbit
+x = x + 1
+```
+
+changes to
+
+```moonbit
+x <~ x + 1
+```