[2/n] - Remove code-size-costly optimizations

[Dirbaio]

Part 2 of N of #492. Depends on #505

• Remove bool compression
• Remove LEB128 compression. usize/isize are now 4 bytes, format tags are now 2 bytes.

Code size comparsion (only including gains from this PR, not #505):

                             before    after    change
debug:                       107232   101452    -5.3% 
release:                      40852    37396    -8.4%
release + flags:              21936    19416   -11.4%
release + flags + buildstd:   20524    18004   -12.3%

rustc 1.54.0-nightly (676ee1472 2021-05-06)
"flags" means these in Cargo.toml:
    codegen-units = 1
    debug = 2
    debug-assertions = false
    incremental = false
    lto = 'fat'
    opt-level = 'z'
    overflow-checks = false

"buildstd" means these in .cargo/config.toml:
    [unstable]
    build-std = ["core"]
    build-std-features = ["panic_immediate_abort"]

[jonas-schievink]

Looks good!

[Urhengulas]

The same applies for this PR, as for #505 :)

[jonas-schievink]

bors r+

[bors]

Build succeeded:

• ci