Introduce Undefined Value Graphs (UVGs)

[Akuli]

The purpose of UVGs is to eventually fix issues like #3, #445 and #566. This PR only adds code that builds UVGs, and doesn't use them for anything yet.

An UVG is somewhat similar to LLVM IR and the CFGs in old Jou compilers, but much simpler. Currently, it only contains three instructions:

• set x: A value is stored to variable x.
• use x: A value is loaded from variable x.
• don't analyze x: Something more complicated is done with variable x. Compiler doesn't fully understand what, so it will not complain.

For example, consider the following function (taken from docs included in this PR):

import "stdlib/io.jou"

def foo(a: int) -> int:         # line 3
    x = a + 6                   # line 4
    y: int                      # line 5
    z = y                       # line 6
    printf("%d %d\n", x, y, z)  # line 7

Running jou --uvg-only file.jou prints the following UVG:

===== UVG for foo =====
block 0 (start):
    [line 3]   set a
    [line 4]   use a
    [line 4]   set x
    [line 6]   use y
    [line 6]   set z
    [line 7]   use x
    [line 7]   use y
    [line 7]   use z
    [line 7]   use return
    Return from function.