* Rewrite the error cost comparison in terms of explicit, discrete
conditions.
* Allow merging versions have different error costs.
* Store the depth of each stack version since the last error. Use this
state to prevent incorrect merging.
* Sort the stack versions in order of preference and put a hard limit on
the version count.
This bumps the size of the reference counts from 16- to 32-bit counters to make
it less likely to overflow. Also assert in the retain function that the
reference count didn't overflow.
32-bits seems big enough for non-pathological examples but a more fool-proof
fix may be to bump it to 64-bits.
Previously, it was possible for references to external token states to
outlive the trees to which those states belonged.
Now, instead of storing references to external token states in the Stack
and in the Lexer, we store references to the external token trees
themselves, and we retain the trees to prevent use-after-free.
The parser spends the majority of its time allocating and freeing trees and stack nodes.
Also, the memory footprint of the AST is a significant concern when using tree-sitter
with large files. This library is already unlikely to work very well with source files
larger than 4GB, so representing rows, columns, byte lengths and child indices as
unsigned 32 bit integers seems like the right choice.
Before, any syntax error would cause the lexer to create an error
leaf node. This could happen even with a valid input, if the parse
stack had split and one particular version of the parse stack
failed to parse.
Now, an error leaf node is only created when the lexer cannot understand
part of the input stream at all. When a normal syntax error occurs,
the lexer just returns a token that is outside of the expected token
set, and the parser handles the unexpected token.
