Support Worker Function Invocation

[afsalthaj]

Fixes #442

Main changes

• Allows a Rib script that can consist of worker invocation function to form the response

let status = 200;

let start_time = request.path.time;
let end_time = request.path.end_time;
let result = timeline:driver/api/run(start_time, end_time);

{ body: result, status: status }

• Make EvaluationContext independent of API Gateway use case. This implies, by the time this PR is ready, at least we can say, Rib is fully independent of Golem OSS API Gateway.

Now evaluation-context is

use golem_wasm_ast::analysis::AnalysedFunction;

#[derive(Clone)]
pub struct EvaluationContext {
    pub variables: Option<TypeAnnotatedValue>,
    pub analysed_functions: Vec<AnalysedFunction>,
}

• Make evaluator be able to run the invocation expressions in the Rib Expression Tree
  Functions can exist anywhere in the Rib expression tree, and evaluator will execute it. This is based on an idea I got from John that I went ahead with this. Whether to disallow more than 1 function or not, I am leaving with the team.

• Use double-quotes instead of single quotes (this is a decent change from tokeniser itself) to align with wasm-wave syntax. This will make sure we support all of wasm wave syntax

• Allow users to form variants/enums etc which are simple identifiers until evaluation. Depending on the info in evaluation context, they can be resolved to Enum, Variant, or even function-call. It's even better to feed as much as information about types available (like a symbol table) into the execution context in future. With this PR, the door is opened for it.

@jdegoes I thought I will have at least this decent shape before we start pairing up. It's even easier to reason about what's in there, and I will be more comfortable to pair up on this branch to bring in type information and fix things.