Add variable evaluation logic

[dylandreimerink]

A common feature is to be able to perform some basic evaluations on the current program state. This issue proposes to add a eval/exec/call command to the debugger which then will evaluate an expression like local_var_a == struct a{.a=123, .b=234}, global_var_b >= 20 && !global_var_c, or simply local_var_d (which will print its value if it exists).

This has the following purposes/goals:

• Together with the new command, this is an easy way to inspect the current program state.
• These same expressions can be used for conditional breakpoints and logging break points.
• Could be a stepping stone to more complex macros for more advanced automation.

The exact feature set is something to be determined and can be iterated over time, here are some initial suggestions:

• Reference local variables (see #13)
• Reference global variables (see #14)
• Reference registers, perhaps like #R1 or some other prefix which is illegal in C identifiers to avoid conflicts
• Have boolean operators ==, !=, >=, <=, <, >
• Use BTF to match int, string, bool, struct types.
• true and false keywords for boolean comparason
• Round brackets(( and )) to indicate precedence, otherwise left to right.