[Feature] compile time macros

[b1ek]

i think we should allow for some macro-like syntax that is evaluated at compile time to save runtime.

not sure how to implement this, though. i think that they should emit amber code that is inserted into the places where they are called. like this:

macro compile_env(var: Text): Text {
    return text_value(trust $\${var}$);
}

echo compile_env("LANG");

which will evaluate to something like this, assuming that LANG is en on compile time:

echo "en"

text_value() returned a text literal with the value of trust $\${var}$, which was evaluated on compile time

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ref #436 , but this issue is for discussing the implementation of macros themselves, not the compile_env!() macro specifically