switch to dynamic dispatch for TokenSource

switch to dynamic dispatch for TokenSource

Benchmarks show no difference. This is probably because we are
bottlenecked on memory allocations, and we should fix that, but we are
not optimizing for performance just yet.

changes. Lines starting # with '#' will be ignored, and an empty
message aborts the commit. # # On branch token-source # Changes to be
committed: # modified: crates/ra_syntax/src/parsing/parser_api.rs #
modified: crates/ra_syntax/src/parsing/parser_impl.rs #