[backend-prose] Duplicate prose definitions

[rossberg]

I added a check for overlapping anchor definitions, which uncovered 3 cases where the prose backend apparently creates duplicates:

warning: prose rule `exec/array.new_data` has multiple definitions
warning: prose rule `exec/call_ref` has multiple definitions
warning: prose rule `valid/cvtop` has multiple definitions

I looked at exec/call_ref, and the second prose it generates is

1. YetI: TODO: It is likely that the value stack of two rules are different.

[jaehyun1ee]

Please take a look at #69, all warnings are resolved, yet the third case for valid/cvtop needs improvement (possibly via another PR).