stencil: Various enhancements

[jpsamaroo]

This PR improves stencils in a number of ways:

• Removes the need for spawn_datadeps around @stencil
• Allows expressions like @stencil(sum(@neighbors(A[idx], 1, Wrap()))), which auto-allocates the output container
• Validates that @stencil can be used in broadcast expressions
• Supports writing to read variables, like @stencil(A[idx] = sum(@neighbors(A[idx], 1, Wrap())))

Written by Gemini

Todo:

• Update docs