Base documentation - minor revisions

Many revisions on the base.rst. Formated and added new examples, inserted some returned types, etc.
Corrections on base.rst previous commit

Removed type annotation of some functions
Corrected identation and comments
tab substitution of LaTeX symbols in REPL (fixes #6340)
make deserialization backwards compatible (allow reading old data)

make serializing strings even faster
Merge branch 'master' of github.com:JuliaLang/julia
remove duplicate LaTeX symbols (font variations) ... fortunately, it looks like the most reasonable choice is listed first in the unicode.xml file
add \sqrt and \cbrt substitutions
fix #6909
add missing \mars and \female substitutions, for consistency as pointed out by @mbauman
Merge pull request #6911 from stevengj/latex

RFC: tab substitution of LaTeX symbols in REPL
Faster implementation of var across dimensions
Merge pull request #6912 from JuliaLang/sjk/faster-var

Faster implementation of var across dimensions
Fix #6907.

Changes the default p in norm(::SparseMatrixCSC, p) to be p=2, to be consistent with other norm functions.

At the same time, document that norm(::SparseMatrixCSC, p=2) is currently unimplemented. Throw more helpful error message.

Note: the behavior now is that norm(::SparseMatrixCSC) will throw an error by default, as opposed to silently computing the 1-norm.
Merge pull request #6864 from brk00/patch-3

Base documentation - minor revisions
fix #6896, RHS not properly returned on self-assignment
allow more math symbols as identifier characters
fix #6908, document need for sphinx_rtd_theme to build docs
in math symbol identifier whitelist, add comments showing the characters corresponding to the codepoints
parser code reorg; make precedence easier to deal with