Improve lazyexpr syntax from python client

Perhaps by using a context manager one could write code like

```
N = 100
y = linspace(N)
x = sin(y) * 3
```
and have an expression generated with name ``x`` in ``@personal``. This avoids the cumbersome syntax
```
N = 100
y = client.lazyexpr(name='y', expression=f"linspace({N})")
x = client.lazyexpr(name='x', expression="sin(y) * 3", operands={'y': y})
```