BoolExpr should be initializable with C++ boolean values

This code works:
```
LinIntExpr e = 0;
```
but this code fails:
```
BoolExpr e = false;
```

It seems that it should be possible to initialize BoolExpr with a C++ boolean value.

Search the C++ API implementation and existing tests for LinIntExpr and BoolExpr construction, then compare how integer and boolean literals are handled. Confirm the intended BoolExpr forms for true and false, add coverage for those initializations, and verify the existing LinIntExpr example remains valid.

Written by the indexing model from the issue text.