instead use pure substring matching to find template values.

this makes stdZulu unnecessary and allows formats
like "20060102 030405" (used in some internet protocols).

this makes Parse not handle years < 0000 or > 9999 anymore.
that seems like an okay price to pay, trading hypothetical
functionality for real functionality.

also changed the comments on the Time struct to use the
same reference date as the format and parse routines.

R=r
CC=golang-dev
https://golang.org/cl/833045