[Syntax] Allow more type deduction cases

[SirLynix]

Currently, type deduction only occurs for variable declarations:

let x = 42.0;

is equivalent to

let x: f32 = 42.0;

It would be great to allow type deduction for other cases:

let x: f32;
let v = vec3(x, x, x); // no need to write vec3[f32](x, x, x)

Other examples:

let x: mat4[f32];
let y = mat3(x); //< okay, y is a mat3[f32]

let x: f32;
let a = array(x, x, x); // a is an array[f32, 3]

const vertPos = array[vec2[f32]](
	vec2(-1.0, 1.0), //< no need to write vec2[f32] for every value
	vec2(-1.0, -3.0),
	vec2( 3.0, 1.0)
);

It should also be pretty easy to add return type deduction (but i'm not sure it should be done):

fn foo() {} //< returns nothing
fn bar() { return 42; } //< return i32
fn foobar() { return vec3(1.0, 2.0, 3.0); } //< return vec3[f32]