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* Support "modern" declaration syntax as an option
Fixed #202
This change adds four new declaration keywords:
The `let` and `var` keywords introduce immutable and mutable variables, respectively. They can only be used to declare a single variable at a time (unlike C declaration syntax), and they support inference of the variable's type from its initial-value expression.
Examples:
```
let a : int = 1; // immutable with explicit type and initial-value expression
let b = a + 1; // immutable, with type inferred
var c : float; // mutable, with explicit type
var d = b + c; // mutable, with type inferred
```
These declaration forms can be used wherever ordinary global, local, or member variable declarations appeared before. Right now they do not change rules about what is or is not considered a shader parameter. The `static` modifier should work on these forms as expected, but a `static let` variable is *not* the same as a `static const`, so an explicit `const` is still needed if you want that behavior.
A `typealias` declaration introduces a named type alias, similar to `typedef`, but with more reasonable syntax. It inherits from the same AST class that `typedef` uses, so all of the code after parsing should be able to treat them as equivalent. To give a simple example:
```
// typedef int MyArray[3];
typealais MyArray = int[3];
```
A `func` declaration introduces a function. Like `typealias` it re-uses the existing AST class, so there is no need for major changes after parsing. A `func` declaration uses a syntax similar to `let` variables for its parameters, and takes the (optional) result type in a trailing position. For example:
```
func myAdd(a: int, b: int) -> int { return a + b; }
```
If a `func` declaration leaves of the return type clause, the return type is assumed to be `void`.
The main difference (beyond the trailing return type) is that the parameters of a `func`-declared function are immutable (unless they are `out`/`inout`).
This change doesn't add support for declaring operator overloads with `func`, but that should be added later, and I'd like to make that the only way to declare such operations:
```
func +(left: MyType, right: MyType) -> MyType { ... }
```
The use of `:` for declaring parameter types here means that a function declared with modern syntax currently cannot include HLSL-style semantics on its parameters (or its result).
We might consider introducing an `[attribute]`-based syntax for adding semantics to parameters if we think this is important, but for now it is fine to insist that users declare their entry points using traditional syntax.
This change strives to avoid unecessary changes after parsing, but if the new syntax catches on with users there are some small ways we can take advantage of it for performance. In particular, since `let` declarations and parameters of modern-style functions are immutable, we do not need to generate read/write local temporaries for them during lowering to the IR (technically we can make the same optimization for `const` locals).
In the process of implementing these new forms I also added a few subroutines to help share code better between existing cases in the parser. In particular, parsing of generic parameter lists on declarations that can be generic is now simplified and more unified.
* Fixup: remove leftover debugging code
* fixup: typos
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