Enhance grammar handling with linked list support and memory management functions

[ydah]

Overview

This PR adds a complete generic linked list implementation to stdlib.y, enabling users to immediately use list(), nonempty_list(), and separated_list() parameterized rules without writing any boilerplate code.

What Changed

1. Added linked list implementation to stdlib.y

New data structure and functions available via %code requires:

• lrama_list_node_t - Generic singly-linked list node
• lrama_list_new(void*) - Create new list
• lrama_list_append(list, void*) - Append element
• lrama_list_free(list) - Free list structure
• lrama_list_length(list) - Count elements
• lrama_list_get(list, index) - Get nth element

2. Added semantic actions to list rules

Before:

%rule nonempty_list(X)
    : X
    | nonempty_list(X) X
    ;

After:

%rule nonempty_list(X)
    : X { $$ = lrama_list_new((void*)$1); }
    | nonempty_list(X) X { $$ = lrama_list_append($1, (void*)$2); }
    ;

3. Simplified separated_list() implementation

Before: separated_list(sep, X) → option(separated_nonempty_list(sep, X))

• Generated 3 non-terminals: option_..., separated_nonempty_list_..., separated_list_...

After: separated_list(sep, X) directly expands to 3 rules

• Generated 1 non-terminal: separated_list_...
• Fewer symbols, simpler grammar

4. Enhanced stdlib merging in lib/lrama/command.rb

• Merges prologue sections from stdlib.y into user grammar
• Prepends %code directives from stdlib.y before user code
• Ensures helper functions are available when list rules are used