[Polynomial] Simplified polynomial's coefficients retrieving API

Implement API for retrieving coefficients of multivariate polynomials like:
```kotlin
val polynomial: LabelledPolynomial<C> = ...

val coefficient1 = polynomial[{ x pow 3u; y pow 1u; z pow 15u }] // Signature description as in DSL1
val coefficient2 = polynomial[mapOf(x to 3u, y to 1u, z to 15u)] // Just a `get` function delegated by `polynomial` to `polynomial.coefficients`
```