Clippy needless_borrow and single_match warnings fixed

[ggaspersic]

Resolving the following clippy warnings:

• https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
• https://rust-lang.github.io/rust-clippy/master/index.html#single_match

Smaller additional changes:

• output buffer truncated before filled in parser.rs to prevent unnecessary slice creation
• inlined if into else in block_ffm.rs
• simplified gz var cache.rs