Make String#freeze spec-compliant

[seven1m]

#217

Today I learned:

'foo'.freeze.equal?('foo'.freeze) # => true

Interesting to note that it only seems to apply to string literals and doesn't work if you set a variable first:

s = 'foo'
s.freeze.equal?('foo'.freeze) # => false

(Good thing too, because I have no idea how we'd do that with the compiler! 😅)

[herwinw]

(Good thing too, because I have no idea how we'd do that with the compiler! 😅)

Add a hashmap with the frozen strings, in case you freeze a variable that contains a string, look the actual value up in the vector, add it if it doesn't exist, and assign the address of the StringObject from the map to the variable.
I don't think this would be very good for your performance though.

[seven1m]

This was really easy thanks to the work in #2087. 🤗