Add supports for immutable strings

[jcubic]

According to R7RS spec:

Every object that denotes locations is either mutable or
immutable. Literal constants, the strings returned by
symbol->string, and possibly the environment returned
by scheme-report-environment are immutable objects.
All objects created by the other procedures listed in this
report are mutable. It is an error to attempt to store a
new value into a location that is denoted by an immutable
object

So string literals and (symbol->string 'sym) should be immutable.