[fidlc] Sharing error formatting in ErrorReporter

Test: unit tests, no expected behavior change
Change-Id: I2cfd3488ed56e0e887d125b46b545dca26bee17f

[fidl][errors] Move std::string errors into the reporter

Any error message is likely to have dynamic or contextual information,
and thus be an allocated std::string rather than a const char*. Making
the reporting infrastructure just take std::strings accordingly, since
all its callers are making one anyway.

Change-Id: I1165c773ac3c90e6664658b24b0cba6cc899d0d2

[build][fidl] Separate the fidl (host) lib from the compiler binary.

TO-732

Change-Id: I02625c25d3cac7bb7316370c47c765e1a66e9a01

[fidl] Add a notion of SourceLocation, and use it in a bit of error reporting

Change-Id: Ie27a04dcd83d639ca2d4a029773c4d6bdadee1e3