Handle side-effects in expression evaluation

[djeedai]

Ensure that expressions with a side-effect, like rand(), are evaluated only once even if they're referenced multiple times in a module. Otherwise each evaluation duplicates the side effect, which in the example of rand() yields a different value even though the expression is the same.

Add Expr::has_side_effect() to determine if an expression has any side effect. Currently only expressions returning a random value have a side effect.

To ensure a unique evaluation per expression, add an evaluation cache to the implementations of EvalContext, saving the evaluated string associated with an ExprHandle.

Add to EvalContext some helpers:

• make_local_var() to create a unique variable name, for storing intermediate evaluations.
• push_stmt() to write the statement which actually stores the evaluation of an expression into a local variable before it's referenced.

Remove the module() and expr() from EvalContext, and move the mutable Module reference out of that context, passing it explicitly instead. This allows acquiring a mutable reference to both the context and the module at the same time, which is not possible if the module reference is stored as a context field.

Add EvalContext::make_fn() to generate a function with a temporary function-local context. This ensures that functions which use expressions with side effects have their local statements emitted inside the body of the function, and not at the call site where they're not available.

Fix several bugs in some older modifiers which were using hand-crafted code not properly parenthesized, which could lead to invalid expression evaluation due to operator priorities in WGSL:

• SetPositionCircleModifier
• SetPositionSphereModifier
• SetVelocityCircleModifier
• SetVelocitySphereModifier
• SetVelocityTangentModifier

Bug: #240