Split Constant State Variables doc

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+.. index:: ! constant
+
+************************
+Constant State Variables
+************************
+
+State variables can be declared as ``constant``. In this case, they have to be
+assigned from an expression which is a constant at compile time. Any expression
+that accesses storage, blockchain data (e.g. ``now``, ``address(this).balance`` or
+``block.number``) or
+execution data (``msg.value`` or ``gasleft()``) or makes calls to external contracts is disallowed. Expressions
+that might have a side-effect on memory allocation are allowed, but those that
+might have a side-effect on other memory objects are not. The built-in functions
+``keccak256``, ``sha256``, ``ripemd160``, ``ecrecover``, ``addmod`` and ``mulmod``
+are allowed (even though, with the exception of ``keccak256``, they do call external contracts).
+
+The reason behind allowing side-effects on the memory allocator is that it
+should be possible to construct complex objects like e.g. lookup-tables.
+This feature is not yet fully usable.
+
+The compiler does not reserve a storage slot for these variables, and every occurrence is
+replaced by the respective constant expression (which might be computed to a single value by the optimizer).
+
+Not all types for constants are implemented at this time. The only supported types are
+value types and strings.
+
+::
+
+    pragma solidity >=0.4.0 <0.6.0;
+
+    contract C {
+        uint constant x = 32**22 + 8;
+        string constant text = "abc";
+        bytes32 constant myHash = keccak256("abc");
+    }