Ruby AST: s/no_comments/base/

Nodes are generally supposed to define #to_ruby_no_comments method,
which emits core of the Ruby code of the node. The more general #to_ruby
method then mechanically wraps this code with comments, if the node has
any.

This commit changes the name from #to_ruby_no_comments to #to_ruby_base.
This is because #to_ruby may not do only comment-processing in the
future and the old name wouldn't be accurate in that case. The new name
is also a bit shorter.

Similar reasoning holds for #single_line_width_without_comments, which
gets renamed to #single_line_width_base.
Ruby AST specs: Make description of context-checking tests more generic

This commit changes test descriptions like

  it "passes correct available space info to #to_ruby"

to

  it "passes correct context to #to_ruby"

This is because these tests may also check other things beside space
info in passed contexts in the future. The new way is also a bit
shorter.