`Trapdoor` update and new methods for `DCRTPoly`

[enricobottazzi]

The main contributions of the PR are:

• Refactor of the Trapdoor methods
• Addition of the Trapdoor generation and gauss sampling for square matrix target
• Support for sampling operations for DCRTPoly from gaussian, binary and ring uniform distributions
• Support for basic arithmetic operations (add and mul) for DCRTPoly
• Support for creating a DCRTPoly starting from a vector of rust strings/bigintegers