Stop checking if values are symbols

[okamsn]

We currently assume that if a value is a symbol (as in a variable name), then we don't need to capture the value in a variable. However, if that variable name is actually a macro symbol from cl-symbol-macrolet, then that might not be true.

It is safer to just always capture the value.

[okamsn]

The answer is to only pass through values if they are constants, as done in macroexp-let2*. We will add a loopy--instr-let2* which works similarly, pass through constants if possible and creating variable-binding instructions when needed.

[okamsn]

Done in #180.