Constructs new Atom using MeTTa S-expressions syntax for expressions. Recognizes grounded
strings, numbers and booleans. For other grounded symbols use braces.
Macros has a performance penalty because it creates and uses an additional
wrapper for grounded atoms.
Similar to metta! but constructs a constant Atom. Main goal is to be able
writing const SOME_SYMBOL: Atom = metta_const!(SomeSymbol). This macros
uses constant constructors for symbols, variables and expressions internally.
Grounded values are not supported as they cannot be instantiated without
allocating memory.