Add meaningful #inspect to classes

[zverok]

Imagine irb session:

SymEngine::Symbol.new('x')
# currently:
# => #<SymEngine::Symbol:0x8571680>
# should be:
# => #<SymEngine::Symbol(x)>
SymEngine::Symbol.new('x') + SymEngine::Symbol.new('y')
# currently:
# => #<SymEngine::Add:0x842e0d4>
# should be:
# => #<SymEngine::Add(x+y)>

...and so on.

I assume, pretty simple code, wrapping #to_s into common for Ruby #<#{self.class}(#{to_s})> will play, at least for the start.

[zverok]

Implemented it myself in #44