A struct declaration with variables is now absorbed into the 'declaration'
rule, like any other variable declaration.
A struct declaration without variables is now reduced to the
'struct_declaration_without_vars' rule.
They both are reduced to a 'declaration_statement' in the end.
In a declaration with multiple variables, the variables must be created
before the initializer of the next variable is parsed. This is required
for initializers such as:
float a = 1, b = a, c = b + 1;
A requisite for this is that the type information is parsed in the same
rule as the first variable (as a variable_def_typed) so it is
immediately available to declare the first variable. Then, the next
untyped variable declaration is parsed, and the type from the first
variable can be used to declare the second, before the third is parsed,
and so on.
Basically, declare_vars() is separated in three functions:
1. check_invalid_in_out_modifiers(), which is to be called once per
declaration and emits an error when in or out modifiers are used for
these non-parameter variables.
2. declare_var(), which now handles one variable at the time and doesn't
free any memory.
3. initialize_vars(), which takes care of preparing the initialization
instructions of several variables and frees their struct
parse_variable_def, using exclusively free_parse_variable_def().
This allows to declare variables individually before the initializer of
the next variable in the same declaration is parsed, which is used in
the following patches.
Also, simplifies memory management.
