Rollup merge of #43098 - alexcrichton:more-proc-macro, r=jseyfried
Rollup merge of #43098 - alexcrichton:more-proc-macro, r=jseyfried

Add `isize` and `usize` constructors to Literal

This commit fills out the remaining integer literal constructors on the
`proc_macro::Literal` type with `isize` and `usize`. (I think these were just
left out by accident)

GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust