Build signatures without trivia

[piotmag769]

Currently signatures in scarb-doc sometimes contain trivia, particularly comments. Example:

struct MyStruct{
  /// Field.
  field: u8
}

This happens because the logic of fetching the signatures is basically getting all the text from the node with item definition. We should instead: traverse the subtree of the node, get text without trivia from each leaf, concatenate it together (remember about adding whitespaces where neccesary) and format it.

Note: if we decide to do #1426 first/in near future then this task becomes irrelevant