TAIDL-TO v2.8

Changes since v2.7 (the operation set and its reference in docs/operations.md, plus a README line on the examples and the Docker scripts' --cpu and --cuda13 flags):

• Lowered constant and iota (#112)
• Lowered the elementwise operations TAIDL v2.13 lexes (#114)
• Lowered compare, select and clamp (#116)
• Lowered the seven structural operations, including slice and gather (#118)
• Lowered the six other reduces, each starting from its identity (#120)
• Lowered cholesky, triangular_solve and batch_norm_inference (#122)
• Added the operations reference under docs (#123)
• Described what the example accelerators model in the README (#124)
• Let the docker scripts take --cpu and --cuda13 (#126)

What changes for users:

• generate_oracle lowers every operation word that TAIDL v2.13 lexes to the element types that docs/operations.md lists for it.
• It refuses with ValueError a constant literal outside its type's range, a compare whose type does not fit its operands, and a batch_norm_inference epsilon past the range of f32, with NotImplementedError an index of dynamic_slice, dynamic_update_slice or gather that is not of an integer type; and with TypeError a constant number that is not an integer at an integer type, or a pred literal at another type.
• A list attribute or a slice range may contain an expression in backticks, which is evaluated at each call.
• The Docker scripts take --cpu or --cuda13, which overrides the image picked from the host.

This release:

• The version is 2.8 in pyproject.toml, taidl_to/__init__.py and the README's install line.
• README.md gains an Operations section on what an oracle lowers and on the tests' bounds, and it names the refusals that the lowerings add.
• The release still pins TAIDL v2.13.