Lib.rs

reuse rustc_errors for compiler_base.

note: [WIP] Do not use it.

lib.rs:

Text rendering and related helper functions.

Reuse 'styled_buffer.rs' in 'rustc_errors', and 'styled_buffer.rs' has been modified to fit the feature of 'Compiler-Base'.

• add method appendl() and pushs() to StyledBuffer.

• replaced the enum Style with generics T: Clone + PartialEq + Eq + Style to support extending more styles. StyledBuffer still should be valid when facing the user-defined style, rather than just supporting a built-in enum Style.

• add some test cases for 'StyledBuffer'.