Avoid identifier naming collision in getter, setter, and property macros

[jgaskins]

TL;DR

Currently, you can't refer to an identifier called value inside a getter, setter, and property blocks due to that identifier colliding with one defined inside the macro. This commit uses a safe variable name to avoid that collision.

Details

Given the following code:

struct Foo
  getter value = "bar"
  getter baz : String do
    value.upcase
  end
end

Foo.new.baz

I get this error:

In foo.cr:8:9

 8 | Foo.new.baz
             ^--
Error: instantiating 'Foo#baz()'


In foo.cr:4:11

 4 | value.upcase
           ^-----
Error: undefined method 'upcase' for Nil

Nil trace:

  foo.cr:4

        value.upcase


  macro getter (in expanded macro: macro_4331394032:117):10

                if (value = @baz).nil?


  macro getter (in expanded macro: macro_4331394032:117):10

                if (value = @baz).nil?


  macro getter (in expanded macro: macro_4331394032:117):7

              @baz : String?

This didn't make sense because my value method is guaranteed to return a String at compile time. One thing I noticed was that changing the getter baz block to self.value.upcase worked, which pointed to a naming collision.

I did find local variables named value in those macros, and this PR updates them with the % sigil. With this PR, that example code runs as expected.