Tail call optimization for mutually (and directly) recursive functions in Ruby.
The current desing uses a trampoline. However, it is implemented in a way that still allows a tail recursive function to return a Proc as its terminal value without it being called prematurely by accident.
### examples
```ruby
require 'mutual_recursion'
def mutual_one(x, y = 0)
return terminal_value(y) if x.negative?
tail_call { mutual_two(x, y + 1) }
end
def mutual_two(x, y)
tail_call { mutual_one(x - 1, y) }
end
mutual_one(50_000).invoke
# => 50001
```
```ruby
require 'mutual_recursion'
def direct(x, y = 0)
return terminal_value(y) if x.negative?
tail_call { direct(x - 1, y + 1) }
end
direct(50_000).invoke
# => 50001
```
```ruby
require 'mutual_recursion'
def proc_returning(x, y = 0)
return terminal_value(proc { "|#{y}|" }) if x.negative?
