introduce SchemaEntityBuilder

Introduce `SchemaEntityBuilder`, inspired by Scala’s `Builder` type in the collections library.

`SchemaEntityBuilder` is intended as an alternative to `Props`. It seems simpler, and less syntactically noisy compared to using `Props`.

Example usage:
```scala
val issuer = (
    SchemaEntity.newBuilder
      += (name       -> "Issuer")
      += (balance    -> BigDecimal(0))
      += (minBalance -> BigDecimal(-1000))
  ) withId DId(Partition.USER)
```

Note that the enclosing parens are required as `+=` is an ‘assignment’ operator and thus is parsed with the lowest precedence.