This extends the `LibertyInputStream` added in the previous commit to
allow arbitrary lookahead. Then this uses the lookahead to find the
total length of the token within the input buffer, instead of consuming
the token byte by byte while appending to a std::string. Constructing
the std::string with the total length is known avoids any reallocations
from growing std::string's buffer.
The lexer for liberty files was using istream's `get` and `unget` which
are notorious for bad performance and that showed up during profiling.
This replaces the direct `istream` use with a custom LibertyInputStream
that does its own buffering to provide `get` and `unget` that behave the
same way but are implemented with a fast path that is easy to inline and
optimize.
