Disable READ_CHAR echoing

Add READ_CHAR instruction and read_char statement

Signal error for machine version mismatch

Add AltCover dependency to Assembler.Tests

Compute the test coverage as follows:
    Assembler.Tests$ dotnet test /p:AltCover=true

Output file: coverage.xml

Update F# packages

Correct typo in sigx optimization

Minor improvement

Clear high bits during sign extension

This closes #33.

Optimize constant signed right shift

Solve issue with inserted NOPS