Support substitution

[odashi]

Overview

Add codegen rules for subscription x[i][j][...] -> $x_{i,j,...}$.
CC: @MAKMED1337

Details

Codegen strategy:

1. Converts nested ast.Subscript to a tuple of the root value and the sequence of indices.
2. Applies the codegen rule {value}_{", ".join(indices)}

References

Fixes #52

Blocked by

NA

[odashi]

Test is failing due to syntax incompatibility in some versions. Fixing it

[ShigekiKarita]

LGTM. Maybe latexify should provide a flag to opt-out this like use_raw_function_name (for example use_raw_subscript).