Add Lifetime span getter and setter

[dtolnay]

This matches the span and set_span accessors we provide on all the various Lit variant types.

Lifetimes are ordinarily treated as a single token. Only the proc macro API splits them into ' punctuation and ident. Consequently the span of those two things is normally identical, except in pathological case when manually constructed from within a proc macro with differing spans. Thus, requiring macros to decide whether to write lifetime.apostrophe.span or lifetime.ident.span() for that span makes the code misleading. The new accessors reflect the fact that really there is only one span involved.

For example rustc parsers 'a as the following TokenStream, in which byte range 95..97 covers both characters of the entire 'a token.

TokenStream [
    Punct {
        ch: '\'',
        spacing: Joint,
        span: #0 bytes(95..97),
    },
    Ident {
        ident: "asdf",
        span: #0 bytes(95..97),
    },
]