v.eval: support of more expressions/statements

[StunxFS]

This PR adds support for more expressions/statements:

• Support options.

Statements:

• ForCStmt: for i := 0; i < 10; i++ { println(i) }

Expressions:

• Comptime if.
• !, && and || boolean operators.
• unsafe blocks.
• nil value.
• none value.
• dump expr.
• AtExprs, @FILE, etc.

Functions:

• exit and print_backtrace.