Changes behaviour of round to use default rounding mode

This makes the changes discussed in  #8750.
We currently have:
```julia
const RoundNearest = RoundingMode{:TiesToEven}()
const RoundToZero = RoundingMode{:TowardZero}()
const RoundUp = RoundingMode{:TowardPositive}()
const RoundDown = RoundingMode{:TowardNegative}()
const RoundFromZero = RoundingMode{:AwayFromZero}() # mpfr only, not in IEEE754
const RoundNearestTiesAway = RoundingMode{:TiesToAway}() # C-style rounding
const RoundNearestTiesUp = RoundingMode{:TiesToPositive}() # Java-style rounding, not IEEE754
```

Option (a): copy IEEE names
```julia
const RoundTiesToEven = RoundingMode{:TiesToEven}()
const RoundTowardZero = RoundingMode{:TowardZero}()
const RoundTowardPositive = RoundingMode{:TowardPositive}()
const RoundTowardDown = RoundingMode{:TowardNegative}()
const RoundAwayFromZero = RoundingMode{:AwayFromZero}() # mpfr only, not in IEEE754
const RoundTiesToAway = RoundingMode{:TiesToAway}() # C-style rounding
const RoundTiesToPositive = RoundingMode{:TiesToPositive}() # Java-style rounding, not IEEE754
```

Option (b), use simpler names
```julia
const RoundNearest = RoundingMode{:Nearest}()
const RoundToZero = RoundingMode{:ToZero}()
const RoundUp = RoundingMode{:Up}()
const RoundDown = RoundingMode{:Down}()
const RoundFromZero = RoundingMode{:FromZero}() # mpfr only, not in IEEE754
const RoundNearestTiesAway = RoundingMode{:NearestTiesAway}() # C-style rounding
const RoundNearestTiesUp = RoundingMode{:NearestTiesUp}() # Java-style rounding, not IEEE754
```

Option (c): same as (b), except for the last two, we could copy [Python which uses "half" instead of "ties"](https://docs.python.org/2/library/decimal.html#decimal.Context):
```julia
const RoundHalfAway = RoundingMode{:HalfAway}() # C-style rounding
const RoundHalfUp = RoundingMode{:HalfUp}() # Java-style rounding, not IEEE754
```